A metered-dose inhaler (MDI) is a device used to deliver medication directly to the lungs. It contains a pressurized canister that releases a specific dose of medication in aerosol form when the user presses down on it. MDIs are commonly used to treat respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD). To use an MDI effectively, the user should shake the inhaler, exhale fully, and then place the mouthpiece in their mouth. As they inhale deeply, they press the canister to release the medication. Proper technique ensures that the medication reaches the lungs for optimal effect.