inhalation route

The inhalation route delivers a drug as a gas, vapour, or fine aerosol that you breathe into the lungs. The lungs are a remarkable absorbing organ: their branching airways open into hundreds of millions of tiny air sacs (alveoli) whose combined surface is roughly the size of a tennis court, sitting against an extremely thin film of blood. A drug that reaches this surface can cross into the blood almost as fast as an intravenous injection.

Inhalation serves two distinct purposes. First, for lung diseases such as asthma, it delivers the drug straight to its target — the airways — so a small inhaled dose works locally while minimizing the amount entering the rest of the body, reducing side effects. Second, for volatile or gaseous agents, it provides rapid systemic effect, which is why general anaesthetic gases and oxygen are given this way, and why inhaled (and smoked) recreational drugs act so quickly.

Because absorption is so fast and avoids first-pass metabolism, the inhalation route gives a near-immediate onset for systemically acting agents. For inhaled gases, the depth and rate of breathing also let the dose be adjusted minute by minute, which is invaluable in anaesthesia.

The downsides are practical: getting the right dose to the right place requires good inhaler technique and proper particle size (too large and it lodges in the throat; too small and it is breathed back out), the lungs can be irritated, and only suitable formulations work. Much of a poorly used inhaler dose is swallowed instead of inhaled.