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How to Whiten Teeth: The Best Teeth Whitening Products in the UK

Contributors:  
smiling lady with white teeth
A whiter smile can bring confidence

Have you ever looked at yourself in the mirror or in a photo and wished your smile was a little bit… brighter? If so, you're hardly alone. Teeth whitening is big business in the UK and there is a vast array of different solutions now available – from state-of-the-art laser treatment to home whitening kits.

But this immense amount of choice can be pretty overwhelming, especially if you're looking at how to whiten your teeth for the first time. To make matters worse, some of the products and cosmetic dental treatments you see advertised online are ineffective or even unsafe.

In this guide to UK teeth whitening methods and products, we'll talk you through the many different options when it comes to getting a whiter smile (including the ones that are a waste of time and/or money). We'll cover a full range of professional and DIY solutions, telling you how much these teeth whitening options cost and how effective they are, so you'll be better informed when it comes to choosing the best way to get whiter teeth.

A brief history of tooth whitening

The question of how to whiten teeth is one that has bothered people for thousands of years.

early dentist
And after this, a trim, sir?

In ancient Egypt, having white teeth was a sign of wealth. Egyptians used a paste made from ground pumice stone and wine vinegar, applied with a frayed stick (the original toothbrush). But that's not as bad as the Romans, who used urine. It would have worked, because it contains ammonia, but thankfully we've now found other solutions.

In the 17th Century, before dentists existed in their own right, barbers would take care of oral health. This often involved pulling problem teeth, but they also whitened teeth by filing down the stained enamel and applying nitric acid. It goes without saying that you shouldn't try this at home.

Fortunately, in the 1960s, dentists identified the whitening properties of peroxide, and 20 years later commercial tooth whitening solutions emerged. Dental trays were filled with a thick whitening gel containing carbamide peroxide – a technique still widely used today. Aren't you glad we have waved goodbye to metal files, urine, and sticks?

How to get white teeth

There are many tried and tested ways to whiten teeth these days, including:

whiten teeth using coconut oil
Coconut oil is a natural way to whiten teeth
  • gels
  • strips
  • trays
  • bleach
  • lasers
  • toothpaste
  • coconut oil
  • charcoal

Confused? Don't worry, all will become clear.

The best teeth whitening method for you will depend on how much you have to spend, how quickly you want to see results, how permanent you want it to be, and how sensitive your teeth are.

Note that all the whitening techniques we cover here will only work on your natural teeth; any dentures, fillings, veneers or implants will remain the same colour.

Causes of discoloured teeth

Yellow dentin shows through white enamel
Yellow dentin shows through white enamel

Have you thought about why you don't have pearly white teeth in the first place? Actually, our teeth are not supposed to be brilliant white. Although the enamel that coats teeth is a blueish-white colour, its translucency means the yellow colour of the dentin below can show through.

Enamel thins as we get older, so it's normal for teeth to darken or become yellower over time. You can read more about why teeth go yellow and what to do about it in our full article on the topic. Medicines like tetracycline can also permanently discolour tooth enamel.

Foods that stain teeth

For most people, the yellowing process is accelerated by the things we eat and drink, and other external factors. These include:

  • Berries: Although the antioxidants in berries provide health benefits, their deep hue can cause staining (have you ever tried to get a raspberry stain out of a white shirt?).
  • Sauces: Brightly coloured sauces like curries and tomato sauce can also contribute to staining, and the acidity of tomatoes makes them a double culprit. Opt for a lighter, creamy version to lessen the chances of tooth staining.
  • Coffee: The dark colour and acidity of coffee both have a detrimental effect on your teeth.
  • Tea: Black tea in particular can be more damaging than coffee. Even lighter herbal teas can erode enamel and cause staining.
  • Wine: The tannins in red wine are responsible for turning teeth a shade of grey. White wine, meanwhile, although not directly responsible for staining, can make existing stains darker.
  • Sugary drinks: It's well known that sugary soft drinks cause tooth decay, but because they do this by wearing away that bright tooth enamel, your teeth will appear darker over time, too.
  • Smoking: The nicotine and tar in tobacco can stain teeth very quickly. Over the years, heavy smokers may find their teeth turning brown.
  • Braces: While braces help improve your smile by making your teeth straighter, at the same time they can leave them looking less-than-white.
yellow teeth culprits
Your favourite drinks could be staining your teeth

You might say that the best way to whiten teeth, then, is just to avoid the things that are likely to stain them. But we know it's not always that simple, so while you sip that coffee we'll continue explaining how to undo the damage later.

One option is to minimise the staining effects of certain foods by eating them alongside foods that can prevent tooth staining such as cheese, lettuce, celery, carrots, apples and cauliflower. These either help clean teeth through friction or work to neutralise the acid which damages enamel.

Of course, brushing your teeth correctly and flossing between them every day will help remove plaque and staining. Even with all these measures in place, there may still come a time when you want to get whiter teeth.

Best teeth whitening treatments in the UK

We've already established that the best way to whiten your teeth will depend on a number of personal factors, including:

  • Your budget
  • The current condition of your teeth
  • How many shades of whitening you want to achieve
  • Whether you want whiter teeth instantly or gradually
  • How much time you want to spend whitening your teeth

The table below summarises the main options, but below you can read more detail about each of these ways to whiten teeth.

Whitening treatment

Pros

Cons

In-office laser whitening

Excellent results within 1-2 hours

May fade quickly; very expensive

Take-home bleaching kits

Strong results; may be combined with in-office treatment

Take 2-6 hours per day; high cost

Home teeth whitening kits

Convenient and affordable; may use LED technology

Results not as strong as professional solutions, depending on ingredients

Whitening strips

Convenient and affordable

Weak results; uneven coverage

Whitening pens

Convenient and affordable

Weak results

Whitening toothpastes

Convenient and affordable

Weak results; may be abrasive

Professional teeth whitening is the most expensive, but you're paying for the expertise of a dental professional who can advise you on any risks and recommend the best whitening treatment for you.

When it comes to any other whitening products you might purchase, there is a great variation between brands in terms of ingredients, technology and results. We recommend you view our individual product review articles, linked in the text below, for further details.

For more specific information about prices, view our teeth whitening costs article, or for more information on the average costs of dental treatment across the UK, view our full report here.

Home teeth whitening kits

Home whitening kits are popular because they are both affordable and convenient. You'll find many different variations on the market, but they all contain a teeth whitening gel which you apply to your teeth, either directly or via a mouth tray.

checking teeth shade
Whitening progress is measured by shade

Application times vary between about 10 and 30 minutes per day, and a single treatment course is usually 1-2 weeks. Home whitening makes it easy to whiten your teeth without too much disruption to your day.

Here are some things to consider when choosing an at-home teeth whitening kit:

  • Where is it made? Places like the UK, US and EU have stricter regulations governing the ingredients used in whitening gels than some other countries. It's always a good idea to check the ingredients list so you know what you're putting in your mouth, and find out where the product was manufactured so you know which regulations it conforms to.
  • LED whitening kits: Also known as ‘blue light whitening kits', these come with an LED light that accelerates the bleaching agent. Bleaching kits without an LED light may contain an accelerator fluid instead, which you apply to your teeth before the gel.
  • Fixed shape vs. mouldable trays: Some kits come with fixed shape mouth trays; others have thermoforming trays which must be placed in boiling water and then moulded to the shape of your teeth. The idea of mouldable trays is that they provide a better fit to ensure the gel stays on your teeth. However, many people find the moulding process difficult to master and prefer ready-to-use fixed shape trays.
  • How is the gel applied? It used to be quite standard that you received your whitening gel in syringes and squeezed it into the mouth tray before pressing it onto your teeth. The problem with this method is that excess gel can easily leak out of the sides and onto your gums, resulting in burns or inflammation. Newer styles of teeth whitening kit contain gel pens, which you use to paint the gel directly onto your teeth. Not only does this ensure better coverage and no leakage; it's also a much more economical, so your whitening gel lasts for longer.

Here are two home whitening kits that we recommend. They are both made in the US, ensuring high-quality ingredients that conform with the relevant regulations. They also both use the ‘paint-on' method described above, making them safer and more cost-effective than some of the others out there.

Smile Avenue Professional Tooth Whitening Kit

This high-quality whitening kit comes from Smile Avenue. The kit is made in the US with a gentle but effective whitening gel formula that also contains the remineralizing ingredient hydroxyapatite.

best teeth whitening at home
Our top pick for a home whitening kit

It's really simple to use. Just paint a thin layer of whitening gel onto your teeth, pop in the LED mouthpiece, and plug it into your phone or laptop to activate it. After 16 minutes, it'll turn off automatically and your whitening session is complete.

You get a generous 4 whitening pens in this kit, and when your supply is finished you can purchase affordable refills to maintain your results. It should only take a couple of days for you to start seeing results, but you can continue the treatment for 1-2 weeks for best results. The shade guide provided lets you easily measure your tooth colour before and after.

You can find more information in our Smile Avenue teeth whitening kit review.

For a limited time only, Smile Avenue is offering 10% off this kit when you use our exclusive discount code at the checkout: DENTA10. You also get free shipping as standard.

Snow Teeth Whitening Kit

The Snow teeth whitening system is a big name from the US, but they offer worldwide shipping so you can also purchase it for teeth whitening in the UK. The unique gel contains a combination of hydrogen peroxide and carbamide peroxide for optimal results.

The regular kit plugs into your phone and needs as little as 9 minutes per day to give you whiter teeth.

wireless teeth whitening kit snow
The Snow wireless whitening kit is a completely different way to whiten teeth

Snow also have a premium wireless kit (pictured) which has a waterproof LED mouthpiece that's safe to use in the shower. Teeth whitening doesn't get much more convenient than that!

All Snow's products are made in the US so you know you're getting a certain level of quality. They do cost a bit more than the standard kits you can buy online, but they work out great value when you consider you're getting a year's worth of treatments included with Snow.

You can visit Snow's website to view their full product range and check their best prices to see if it's right for you, or read our full Snow teeth whitening review.

Our in-depth article on home teeth whitening kits has a lot more information on the types of kit and different brands available. Take a look if you want to compare a wider range of products.

Peroxide teeth whitening at the dentist

If you have the money to spend, it's a good idea to visit your dentist to talk about cosmetic teeth whitening. Dentists are permitted to use hydrogen peroxide teeth whitening gel in the UK in much stronger concentrations than are permitted in over-the-counter products. Not only are their treatments effective; they also know how to carry them out safely and can help if you have any side effects.

There are two main types of professional teeth whitening treatment available at the dentist.

Laser or LED teeth whitening

dental whitening process
A rubber seal stops the bleach reaching your gums

Often considered the best teeth whitening procedure in the UK for instant results, this method uses lasers or LEDs to speed up the bleaching effect of a concentrated gel applied to the teeth.

Your dentist will first apply a rubber seal to protect your gums, then will cover your teeth with a teeth bleaching gel – probably containing carbamide or hydrogen peroxide. The light shone on your teeth activates the chemicals in the gel, letting them get to work faster.

Some treatments, like Philips Zoom, involve 3-4 short sessions within a 1-hour appointment. With others, the light is applied continuously.

After a single one-hour treatment, it's possible to get teeth 7-8 shades brighter. However, some of this effect comes from the teeth being dehydrated, and it may fade quickly. With such an intense procedure, there is also greater potential for sensitivity during and following treatment.

Depending on the condition of your teeth and gums, your dentist might recommend a scale and polish before getting your teeth whitening this way. If you have untreated cavities, these will need to be taken care of too.

Professional bleaching trays

Gel for whiter teeth
Kits come with syringes for inserting gel into trays

A cheaper form of whitening offered by dentists uses custom trays to apply professional tooth whitening gel. This gel is usually stronger than the ones provided in DIY kits so can produce better results.

Your dentist will take impressions of your teeth which are used to create custom mouth trays. Once these are ready, you complete your whitening treatment at home. Some professional whitening gels can be left on overnight – handy if you don't have time to wear your trays during the day. You'll need to ask your dentist about the options they offer, though.

Combined in-office and take-home whitening

Some teeth whitening services combine both of the treatments above. Enlighten teeth whitening is one well-known brand in the UK. You first get your custom trays and complete two weeks of teeth whitening treatment at home, then you return to your dentist for a final 40-minute in-office session.

Philips Zoom also offer take-home bleaching trays which you may purchase separately or combine with a chairside session to get your teeth whiter, faster.

Other home whitening solutions

Teeth whitening kits are the most effective way to whiten teeth at home, but there are other products you can use to brighten your smile without visiting a dentist. You can use these alone for gentle results, or use them to maintain the shade you've achieved from another kind of whitening.

Teeth whitening strips

Teeth whitening strips are placed directly onto teeth, so some people find them cleaner and easier to use than gels and trays.

get a whiter smile
A mess-free way to get a brighter smile

Many brands have the appeal of being peroxide-free, but some still rely on peroxide to whiten teeth. Strips usually need to be applied for around 30 minutes and then removed, but it's also possible to buy strips which naturally dissolve after 10-15 minutes of wear.

Whitening strips do a good job of brightening up your front teeth but aren't as effective for back teeth and the uneven bite surfaces. It can be tricky to reach every part of each tooth with the strip, so they sometimes don't whiten teeth consistently.

Still, they are a cost-effective option and can produce good results quite quickly – in as few as seven days. Read our reviews of the best whitening strips in the UK if you want more details.

Teeth whitening pens

Whitening pens are really more like brushes; they consist of a small tube of gel that is applied directly to teeth via a built-in brush.

Pens usually contain peroxide whitening gel, although some peroxide-free brands are available. Users like them because they're so convenient; you can carry them anywhere and use them whenever you have time.

Treatment takes from 1-2 weeks, with 2-4 applications per day lasting anywhere between 10 and 30 minutes, depending on the manufacturer. You probably won't get bright white teeth this way but you might get them a few shades whiter.

Whitening toothpaste and toothbrushes

Perhaps you've seen a whitening version of your favourite toothpaste at the supermarket and thought “It can't hurt to try”. Well the thing is, it can hurt. Literally. Using an abrasive product like this can scratch away your tooth enamel as well as stains. When used over a long period of time (e.g. to brush teeth every day), many users experience increased sensitivity.

teeth whitening toothbrush
Oral-B's 3D White brushes are designed to whiten teeth

Whitening toothpaste only removes surface stains – rather than altering the colour of the teeth themselves – so if your teeth are naturally yellowish, using whitening toothpaste won't change that. However, it can be used to prolong the effects of other whitening treatment.

It's also the cheapest teeth whitening option around, which makes it a popular option for giving your smile a slight boost, although some brands like AP24 cost a lot more than you may be used to spending on toothpaste! We have written a separate guide to the best whitening toothpastes in the UK, so take a look if you're interested in trying a new brand.

Certain toothbrushes (both manual and electric) are also designed to achieve better stain removal, like the Oral-B Pro 2 2500 3D White.

Ways to whiten teeth naturally

If you're happy to trade quick results for a more natural approach, there are several natural teeth whitening techniques that some people swear by. While these won't give you a dazzling white smile overnight, they can be effective at keeping teeth clean and white over the longer term.

  • Coconut oil pulling (swishing a spoonful around your mouth for 10-15 minutes) is a traditional Indian technique for improving oral hygiene. You'll need to rinse and brush as usual afterwards, but you should notice your teeth feel cleaner. Read our full guide to oil pulling for more details..
  • A homemade whitening paste can be concocted using baking soda mixed with water or a very small amount of hydrogen peroxide. Get a runny consistency and then rub onto teeth for 30 seconds after brushing normally.
  • Activated charcoal will turn your whole mouth black but once you wash it off you'll find your teeth are sparkly clean. Available as a powder or toothpaste – read more about this below.
  • Mashed strawberries can be applied directly to the teeth and left for a few minutes – a technique championed by model Tyra Banks. Be sure to rinse well afterwards because the acid in strawberries can damage enamel.
  • Apple cider vinegar can be effective at removing dark stains on teeth. Rub it on your teeth for about a minute using your finger, then rinse with water and brush normally. Continue for at least a month to get good results.

Here is a video explaining more about the increasingly popular oral health treatment of coconut oil pulling:

This week's health hack: how to oil pull. Oil pulling is one of my favorite health hacks for dental health. In this video – I'll go over 4 easy steps on how to oil pull for white teeth and what the benefits of oil pulling are. LINK TO AWESOME COCONUT OIL: http://amzn.to/1VoMLUg (buy through our Amazon affiliate link to support the show!) Timestamps: (0:30) STEP ONE (1:44) STEP TWO (3:30) STEP THREE (4:10) STEP FOUR (5:05) FREE TOOLS -Y O U M A Y A L S O L I K E – 7 NATURAL WAYS TO FEEL BETTER WHEN SICK https://www.youtube.com/watch?v=N1u1Y… VLOG #006 https://www.youtube.com/watch?v=vF4XZ… VLOG #007 https://youtu.be/YiBUHpfQSb8 __ MY PRODUCTS & COURSES: The Ultimate Guide to Keeping A Journal ► http://www.mybestjournal.com YouTube Success Formula (Kindle) ► http://amzn.to/2j86PRU Gear & Resources I Recommend ► http://refusingtosettle.com Hair Product Company I Own/Use ► http://kegleyco.com ✔ SONGS USED ✔ If you like the music in this video be sure you support these artists: Intro + Outro Song “Runaway” and “Then You Came” by Pierce Fulton, follow here: https://soundcloud.com/piercefulton Song “Bastille – Flaws” Remix by Deep Chills, follow here: https://soundcloud.com/deep-chills Song “Thinkin’ Back Pt. 2” by Derlee, follow here: https://soundcloud.com/derleemusic #RefusingtoSettle

Read more about the benefits of using a natural toothpaste and avoiding certain chemicals found in commercial brands.

Activated charcoal

Activated charcoal has natural stain removing properties

Although it might seem counter-intuitive to put something black on your teeth to make them white, there is a good reason why charcoal powders and toothpastes have become so popular. You can read more about whitening with activated charcoal in our separate guide.

Procoal London makes this Activated Charcoal Teeth Whitening Powder which contains 100% coconut charcoal and is a good place to start if you want to give this teeth whitening method a try. You can apply it directly to your teeth with water or mix it with other ingredients to form a homemade toothpaste.

Whitening powders do rely partly on abrasion to remove stains, so it's best to use them with caution. A 60ml tub like this will last 2-3 months with daily use, and longer if you use it less often.

Whitening sensitive teeth

Some patients report experiencing unbearable pain after whitening their teeth. If you have sensitive teeth or have experienced sensitivity in the past, it's a good idea to speak to your dentist about your concerns. He or she may be able to administer topical pain relief to manage any pain you experience after whitening.

If you opt for a home whitening treatment, it might be a good idea to build up to the manufacturer's recommended treatment time. For example, if they tell you to apply a gel for 15 minutes, try it for five minutes first and see how your teeth feel.

Products that don't contain hydrogen peroxide may result in less sensitivity. Potassium nitrate and fluoride are effective pain relievers, while hydroxyapatite (one of the ingredients in Smile Avenue's whitening gel) works to remineralise teeth and block sensitivity.

Tips for keeping your teeth white after treatment

Although there are no truly permanent teeth whitening solutions, there are measures you can take to ensure the effects last as long as possible after you've taken the time to get your teeth shining bright:

  • Cut down on the foods and drinks we mentioned earlier that cause stains in the first place
  • Drink sugary or acidic drinks through a straw
  • Rinse with water or chew sugar-free gum after consuming culprit foods and drinks
  • Don't smoke or vape

Finally, maintain good oral hygiene practices such as regular brushing and flossing. Visit your dentist twice a year for routine checkups, as well as booking a hygienist appointment for a scale and polish. This will help keep your mouth free from the plaque and bacteria that contribute to tooth decay and staining.

Ask a dentist: Why don't our teeth stay bright and white?

Tooth discolouration can be caused due to a number of lifestyle factors, which can change the efficacy of most tooth whitening procedures. For most people, teeth stains are caused by a lifetime of consuming stain-prone food and drinks like coffee and tea. In addition, other stain-causing habits like smoking and chewing tobacco are common culprits, as are certain prescription medications.

For others, it’s not a matter of bad habits but a combination of age and genetics. Teeth whitening treatments can help brighten your smile by several shades; however, going back to these stain-causing habits could affect how long your results last.

Dr. Brandon Murri

Is teeth whitening safe?

The safest way to whiten teeth is through an approved dentist. Dental professionals are trained to carry out various whitening procedures and also to identify cases where whitening isn't recommended.

only let a dentist whiten teeth
Only dental professionals are allowed to whiten teeth by law

For example, people with gum disease, dental caries and dental crowns are advised not to whiten their teeth. For this reason, it's always a good idea to have a dental checkup before doing any kind of teeth whitening procedure.

You may see whitening advertised by local beauty salons or mobile whitening services, but these are not trained dental professionals. In the UK it's illegal for the work to be carried out by anyone who isn't a qualified dentist.

Home whitening kits are safe to use as long as you follow the instructions provided. Don't be tempted to exceed the recommended gel dosage or usage time as this can cause damage to your teeth and gums.

If you're concerned about the risks of whitening your teeth, it's best to speak to your dentist first. You can also talk to people you know who have had similar treatment and ask about their experience.

Checking teeth whitening reviews

Whatever teeth whitening method you choose, you'll be able to find reviews online which give you an idea of what it's like and what kind of results you can expect.

Some brands – especially those offering home whitening solutions – rely on big marketing campaigns and social influencers to create hype around their brand. This doesn't mean they are any less effective, but it's worth searching out some impartial user reviews and checking the scientific studies behind the ingredients if you want to be sure you're choosing the best teeth whitening treatment for you.

Also remember that different people will have different experiences in terms of whitening results, sensitivity, ease of use, and so on. Still, checking reviews from others should help you decide which treatment to have.

Conclusion

There are many teeth whitening options to consider when you want a brighter smile. Visiting a dentist – either for in-office whitening or a take-home kit – will help ensure your treatment is conducted safely and effectively, with the supervision of a trained professional.

However, not everybody can afford professional whitening. In this case, there are many good teeth whitening treatments available over-the-counter and online. They may not be quite as effective as in-office treatments, but they can still whiten teeth by several shades over one to two weeks of treatment.

Teeth whitening products in the UK are heavily regulated, so there is little risk of them damaging your teeth if used as instructed. Whitening gel may be harmful if it comes into contact with gums, which is why we recommend a system like Smile Avenue which gives precise control over where you apply the gel.

Pens, strips and other products can be good for topping up and maintaining results, but may not give much of a whitening effect when used alone.

If you have any concerns about whitening your teeth or are unsure whether it's safe for you, we recommend you speak to a dentist first.

FAQs

How do I find teeth whitening near me?

If a particular brand of professional teeth whitening appeals to you, check the manufacturer's website to find a dentist offering teeth whitening near you. Alternatively, ask your usual dentist if they offer any teeth whitening services, or if they can recommend a nearby dentist who does.

Does teeth whitening work?

Yes, the majority of teeth whitening products work to some degree. But it's important to differentiate between surface stain removal and enamel bleaching. You should also keep in mind that just because a whitening technique works, it doesn't mean it is safe for your teeth in the long run.

How long does teeth whitening last?

Professional bleaching results can last a very long time – even years – if you care for your teeth well afterwards. Teeth whitened with strips and toothpastes might start to yellow more quickly.

Where can I buy hydrogen peroxide teeth whitening gel in the UK?

Dental professionals are the only people licensed to administer hydrogen peroxide whitening gel with concentrations over 0.1% (i.e. anything effective). However, you can buy a kit like Snow, which is made in the US with stronger hydrogen peroxide gel, and ship it to the UK for teeth whitening here.

What are the best products to whiten teeth quickly?

When you get in-office whitening, you'll walk out with noticeably whiter teeth. If your budget doesn't quite stretch to that, try a home whitening kit which should start to whiten your teeth in a few days.

How to Whiten Teeth: The Best Teeth Whitening Products in the UK
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Contributors:
Dr. Brandon M. Murri grew up in Southeastern Idaho. He attended BYU-Idaho, majoring in Biology with Minors in Chemistry and Spanish. He attended Midwestern University’s Dental School in Glendale, AZ, graduating near the top of his class. Dr. Murri works at Family & Cosmetic Dentistry of the Rockies in Fort Collins, CO. Dr. Murri regularly attends continuing education courses and is part of professional groups, like the American Dental Association and the Academy of General Dentistry. In his spare time, Dr. Murri is on the golf course or chasing his boys around the park.
Sources
NHS (National Health Service) https://www.nhs.uk/live-well/healthy-body/teeth-whitening/ Consulted 23rd April 2019. Oral Health Foundation https://www.dentalhealth.org/tooth-whitening Consulted 23rd April 2019. British Dental Association https://bda.org/about-the-bda/campaigns/teethwhitening Consulted 27th May 2020. Dental Protection https://www.dentalprotection.org/uk/articles/tooth-whitening Consulted 23rd April 2019. National Center for Biotechnology Information  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4058574/ Consulted 23rd April 2019.