The idea: skip the gut
Sublingual and transdermal forms are marketed on a simple premise: by bypassing the digestive tract, more of the ingredient reaches the bloodstream. Whether that's true depends heavily on the specific molecule — size, charge, and fat-solubility all determine whether it can cross the mouth lining or skin [1].
Sublingual (under the tongue)
The tissue under the tongue is thin and well-supplied with blood, so some compounds absorb there. Sublingual vitamin B12 is the best-supported example — though notably, swallowed B12 also works well for most people, so sublingual isn't clearly superior unless absorption is a problem (see vitamin B12 deficiency). For many other 'sublingual' supplements, much of the dose is still simply swallowed.
Transdermal (through the skin)
The skin is an effective barrier, which is exactly why transdermal absorption is hard. A few drugs are delivered this way using specialized patches, but most supplement molecules don't cross intact skin in meaningful amounts. 'Magnesium oil' sprays and patches are popular, but evidence that they raise magnesium status comparably to oral magnesium is limited — the oral route is better established (see magnesium types).
Where claims outrun evidence
Many sublingual and transdermal products promise dramatically 'better absorption' without product-specific data. Treat broad bioavailability claims skeptically unless there's evidence for that compound by that route — see bioavailability explained [2].
Practical guidance
- Sublingual B12 is a reasonable option, especially if you dislike pills — but oral B12 usually works too.
- Be skeptical of transdermal vitamin and mineral claims; the skin blocks most of them.
- Look for product-specific evidence, not just the words 'sublingual' or 'transdermal.'
- For nutrients with reliable oral absorption, the fancier route rarely justifies a premium.