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 user presses down on it. MDIs are commonly used to treat respiratory conditions such as 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 users, especially children or those with difficulty coordinating their breath.