Sublingual and Buccal Delivery

Sublingual and buccal delivery both use the oral mucosa, but not the same tissue. The sublingual mucosa under the tongue is thin, non-keratinised and richly vascular, giving fast onset but a small area and constant salivary washout. The buccal mucosa lining the cheek is thicker and less permeable but larger, less mobile and able to hold a mucoadhesive patch. Both drain into the systemic venous circulation without passing the liver first and bypass gastric acid entirely, though permeability still falls sharply with molecular size.

The approved examples make the pattern clear. Sublingual nitroglycerin and buccal fentanyl are small, lipophilic and extremely potent, which is the profile the route suits. Among peptides, sublingual desmopressin works despite an absolute bioavailability of a fraction of a percent, precisely because the systemic exposure required is tiny. Larger peptides have repeatedly failed here, and buccal oxytocin and insulin programmes have produced no approved systemic product.

The route is therefore decided by potency arithmetic rather than convenience. If a compound needs milligram-scale systemic exposure and the mucosa admits well under one percent of what is placed there, no formulation cleverness closes the gap. Variability compounds it, since swallowed fraction, saliva flow and contact time differ between administrations and between people.

The claim to distrust is the sublingual spray or drop sold for compounds studied only by injection. Semaglutide and BPC-157 products in this form have no published mucosal pharmacokinetics, and sellers lean on the true statement that the route bypasses the liver to imply the false conclusion that absorption occurs. Avoiding first-pass metabolism is worthless if the molecule never crosses the epithelium.