The inhalation route

Compare a swallowed tablet and an inhaled mist carrying the same drug. The tablet must first disintegrate, then dissolve, then cross the gut wall, then survive the enzymes and the acid of the digestive tract, and then run the gauntlet of the liver through the portal vein before any of it reaches the general circulation. Each step costs time and removes part of the dose. The inhaled mist faces none of these obstacles. It lands on a surface that is many times larger than the gut’s absorptive area, separated from the blood by a membrane only a fraction of a micrometre thick, and bathed by the entire cardiac output passing through the lungs every minute. The drug dissolves in the fluid lining the alveoli and diffuses straight into the capillary blood. This is why inhalation can produce an effect in two to three minutes while the same drug swallowed takes thirty to ninety minutes. The same anatomy explains the risk: because the route is so efficient and the blood goes from the lungs to the heart and then to the brain before it is filtered anywhere, an inhaled substance reaches the brain almost instantly, and repeated rapid reward is the classic recipe for dependence. Compare this with the gut, where absorption is slower and the drug passes the liver first, blunting the peak. The lesson is that the lung is not merely a local surface but a highly permeable doorway to the whole body, and every inhaled medicine must be judged with that in mind.

The choice between an inhaler and a nebuliser is not a choice between a better and a worse device; it is a choice between two different sets of demands. A pressurised inhaler is small, cheap, quick and portable, so it can be carried everywhere and used at the first sign of trouble. Its weakness is that it requires coordination: the patient must begin to breathe in at the exact moment the device releases the dose, hold the breath, and then wait before the next puff. Children, frail elderly patients, and anyone breathless or confused may fail at that step, and the practical result is that almost no drug reaches the lung even though the prescription is followed. A nebuliser removes the coordination problem: a compressor or an ultrasonic unit turns the liquid into a mist that the patient simply breathes in and out through a mask or mouthpiece over several minutes. That makes it suitable for severe attacks, for patients who cannot use an inhaler, and for drugs that must be given in a large continuous dose. Its costs are that it is bulky, needs a power supply and cleaning, takes longer to deliver a dose, and is more expensive. Between these extremes stand spacer devices, which hold the aerosol cloud so the patient does not have to time the breath, and dry-powder inhalers, which need a strong, deliberate inhalation instead of coordination. The clinical rule that follows is simple: choose the device the patient can actually use, teach the technique, and check it again at every visit, because an inhaled treatment fails silently long before anyone notices that it was never delivered.