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 amount of medication in aerosol form when the inhaler is activated. MDIs are commonly used to treat respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD). To use an MDI effectively, the user must coordinate inhalation with the release of the medication. This ensures that the medicine reaches the lungs properly. Many MDIs come with a spacer, which can help improve medication delivery and make it easier for individuals, especially children, to use the inhaler correctly.