The doorways of the body
When most people picture taking medicine, they picture swallowing a tablet or capsule. The oral route is cheap, safe and familiar, which is why it dominates. But the body has many other surfaces a drug can cross, and each is a genuine route of administration with its own rules. This track covers the routes that go through skin and mucous membranes rather than the gut: across the skin (transdermal), into the lungs (pulmonary), through the nose, eye, lining of the mouth, rectum and vagina.
Think of each surface as a door with a doorkeeper. The skin's doorkeeper is tough and slow; the lung's is thin and fast; the eye lets very little through but you only need a little. A formulator's job is to read the door — its thickness, its wetness, its blood supply — and design a dosage form that gets the right amount through.
Two big reasons to leave the gut
The first reason is the [[phc-first-pass-effect|first-pass effect]]. A drug absorbed from the gut travels straight to the liver before reaching the rest of the body. The liver may chew up a large fraction of the dose, leaving little active drug — poor bioavailability. Routes that drain into the general circulation instead of the liver — under the tongue (sublingual), across the skin (transdermal), into the nose (nasal) — let the drug skip that first pass entirely.
The second reason is local action — delivering the drug where the disease is. An asthma inhaler puts steroid directly onto inflamed airways, so a tiny dose works while sparing the rest of the body from side effects. An eye drop bathes the cornea; a vaginal cream treats a local infection. The same logic explains why a sore-throat lozenge beats a swallowed pill: the drug acts on contact.
Every door has a price
No route is free. The skin is a near-perfect barrier, so only potent, small, somewhat fatty molecules cross it usefully. The lungs are vast and thin but need the drug aerosolised into particles of exactly the right size. The nose and eye flush themselves clean within minutes. Choosing a route is always a trade between how easily the drug gets in and how much the body fights back.
For the rest of this track we will walk through each door in turn: the skin and patches first, then the lungs, then the wet membranes of nose, eye and mouth, and finally the rectum and vagina. Keep two questions in mind for every route — what is the barrier, and why would you choose this door over the mouth?