metered-dose inhaler (MDI)

A metered-dose inhaler is the familiar little pressurized canister-in-a-mouthpiece that puffs out a measured spray of medicine when you press it — the classic “asthma puffer.” Each press releases one precisely metered dose as a fast-moving aerosol you breathe in.

Inside is a sealed canister holding the drug (dissolved or suspended) together with a liquefied propellant under pressure, today usually a hydrofluoroalkane (HFA) that replaced the ozone-depleting CFCs. A metering valve traps a fixed small volume; when you actuate, that volume sprays through a tiny nozzle, the propellant flashes to gas, and the liquid shatters into fine droplets that shrink further as the propellant evaporates, leaving inhalable drug particles. The whole puff happens in a fraction of a second.

The classic difficulty is coordination: the patient must press and inhale slowly at the same moment, or much of the dose splats onto the back of the throat. Spacers and breath-actuated designs help with this. MDIs are compact, portable, deliver a consistent dose, and work without strong inhalation effort, but they require good technique and the propellant itself is a potent greenhouse gas, driving development of greener alternatives.