Why are some pills placed UNDER the tongue?
When someone has an angina attack, a nitroglycerin tablet is placed under the tongue – not swallowed. It takes effect within a minute or so. Why? Because the lining of the mouth absorbs the substance straight into the bloodstream, while a swallowed tablet first has to take the long way round through the stomach, gut and liver before it works.
That detour is called first-pass metabolism: the liver breaks down a large part of the medicine before it reaches the body. The lining of the mouth skips that step entirely. Faster, and less is lost.
The mucosa – a thin, wet, permeable boundary
The difference from ordinary skin is chemical and structural. The outer skin has a thick, horny (keratinised) layer of dead cells that works like a waterproof wall. Large parts of the lining of the mouth – especially under the tongue (sublingual) and on the inside of the cheek (buccal) – are non-keratinised, thinner and woven through with blood vessels, with low enzyme activity. It is roughly the perfect opposite of a barrier.
Permeability isn't even uniform in the mouth. Roughly ranked:
- Under the tongue – most permeable (thinnest, most blood vessels).
- Inside of the cheek – second most.
- The palate – least (more keratinised).
How much passes through depends on the molecule – its size and how fat-soluble it is. Research on the buccal mucosa has estimated an effective "pore radius" of about 1.5–3 nanometres in the barrier, enough for some larger molecules to get through as well.
Medicines that take the shortcut through the mouth
That the mucosa is a shortcut is not a guess – half the medicine cabinet is built on it:
- Nitroglycerin under the tongue for angina – works within minutes.
- Buprenorphine (for pain and addiction) is given buccally or sublingually.
- Fentanyl is available as buccal tablets for breakthrough pain.
- Sublingual immunotherapy (SLIT) – allergen drops under the tongue train the immune system.
- Melatonin and some vitamins are sold as melt-in-the-mouth tablets under the tongue for faster absorption.
What they have in common: the mouth is chosen precisely because it lets substances in quickly and effectively.
Even vaccines can be given through the oral mucosa
It goes further than that. Research on buccal and sublingual vaccines shows that the body's mucosal immune system can be activated through the mouth – without a needle. Such vaccines can be given as thin films or solutions, are often stable at room temperature and could in theory be taken without healthcare staff. The fact that the immune system responds to what meets the mucosa tells us something important: the body treats this surface as an active point of contact with the outside world, not as a dead wall.
What does this mean for everyday substances in the mouth?
Here is the logical bridge – and here we are careful to separate what is known from what is reasonable to wonder about. The mucosa "doesn't check ID". It has evolved to take up substances, and it doesn't perfectly distinguish between "a medicine we chose" and other small molecules or particles it meets every day. That means what you repeatedly rub against your gums actually matters – in a way that doesn't apply as strongly to your ordinary skin.
But size is absolutely crucial. The uptake we have described is about very small things – dissolved molecules and particles at the nano scale (remember the pore radius of just 1.5–3 nanometres). A microplastic particle of tens of micrometres is far too big to pass; only the very smallest nano fraction is even in the right size range. And that very fraction is – as we show in the article on microplastics – the one research can't yet measure (the studies' filters stop at around 0.45 µm). So the bridge has a real data gap in the middle: we know the mucosa takes up small things, and we know toothbrushes shed small things – but exactly how large the overlapping nano amount is has not been measured.
Where the limit of what we know lies: that the mucosa absorbs medicines and responds to vaccines is well established. That microplastic particles from a toothbrush are absorbed through the gums in any significant amount has not been shown or quantified – and the nano size that would be required is below the studies' detection limit. It is a reasonable question based on how the tissue works – not proof. So we are not claiming that your brush leaks plastic straight into your blood through your gums.
A simple conclusion
You can't control everything that meets the lining of your mouth – but your toothbrush is one of the few things you are guaranteed to press against your gums every day, twice a day. Choosing to leave out plastic right there is a cheap, risk-free way to remove one unnecessary variable at an unusually absorbent surface.
Want to understand why plastic bristles shed particles in the first place? Read Microplastics in toothbrushes: the chemistry behind plastic wear.