The nose: fast in, fast out
Nasal delivery has two jobs. Most often it treats the nose itself — a steroid spray for allergy, a decongestant for a blocked nose. But the nasal lining is thin and richly supplied with blood, so it can also send drugs straight into the bloodstream, dodging the first-pass effect; this is how nasal sprays deliver migraine drugs, hormones and emergency naloxone within minutes. A small, much-studied side door even leads from the upper nose toward the brain.
The enemy is mucociliary clearance: tiny hairs sweep the nasal mucus — and any drug riding on it — toward the throat within roughly fifteen to twenty minutes. Whatever has not been absorbed by then is swallowed. So nasal products favour drugs that are potent and quickly absorbed, and may use mucoadhesive gels or thickeners to cling longer.
The eye: a thimble of room
Ophthalmic delivery is almost always local — treating the eye's own surface or front chamber. The catch is brutal: the eye holds only about 7–10 microlitres of fluid, while a standard eye drop is 30–50 microlitres. The excess simply overflows or drains down the tear duct, and blinking flushes the rest within a couple of minutes. As little as 5% of an instilled drop is actually absorbed; the rest is lost.
The mouth: under the tongue and against the cheek
The lining of the mouth is a quiet route with a real advantage: blood from it drains into the general circulation, not through the liver, so it skips the first-pass effect. [[sublingual-delivery|Sublingual]] placement — a tablet or spray under the tongue, where the lining is thinnest — gives the fastest absorption; this is why a glyceryl trinitrate tablet relieves angina in under two minutes. [[buccal-delivery|Buccal]] placement against the inside of the cheek is a little slower but, because the tablet sits still there, suits drugs meant to release over a longer time.
Across all three wet membranes the same theme repeats: the surface is thin and inviting, but the body keeps washing it. The formulator's art is to win the race — make the drug absorb faster, or make the dose stick around longer with mucoadhesive polymers, viscosity or gelling.