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A metered-dose inhaler (MDI) is a device that helps deliver a specific amount of medication to the lungs. It is commonly used to treat asthma, chronic obstructive pulmonary disease (COPD), and other respiratory problems. Because of normal naming conventions, MDIs are considered aerosolized.

An inhaler consists of 2 major components. The canister and a mouthpiece. The canister itself consists a metering dose valve with an actuating stem, with the inside containing the aerosol propellant and medication. The mouthpiece contains the mating discharge nozzle and generally includes a dust cap to prevent contamination.

Once assembled the patient then uses the inhaler by pressing down on the top of the canister, with their thumb supporting lower portion of the mouthpiece, thus releases a mist of medicine that is then breathed into the lungs.

The medication in a metered dose inhaler is most commonly a bronchodilator, corticosteroid or a combination of those two. Other medications less commonly used but also administered by MDI are mast cell stabilizers, such as (cromoglicate or nedocromil).

Through inhalation, it takes only five to 15 minutes for short-acting bronchodilators (quick-relief medicine) to have an effect, compared to oral asthma medicines, which can take one to three hours.^{[citation needed]} With an inhaler, there are also fewer medication side effects, because the medicine goes right to the lungs and not to other parts of the body.^{[citation needed]} A holding chamber (or spacer device) attached to the inhaler can help make taking the medication easier, for adults and children. However, MDIs approved for pediatric use in the US are changing constantly. FDA

Main article: Asthma spacer

Spacers are tubes attached to the inhaler that act as a reservoir or holding chamber. They serve to hold the medication that is sprayed by the inhaler. This makes it easier to use the inhaler and helps ensure that more of the medication gets into the lungs instead of just into the mouth or the air. With proper use, a spacer can make an inhaler 20 percent more effective in delivering medicine.^{[citation needed]}

Spacers can be especially helpful to adults and children who find a regular inhaler hard to use. People who use corticosteroid inhalers should use a spacer to prevent getting the medicine in their mouth, where oral yeast infections can occur.

A metered dose inhaler contains enough medication for a certain number of puffs which is printed on the canister. Even though the inhaler may continue to work beyond that number of uses, the amount of medication delivered may not be correct. It is important to keep track of the number of times an inhaler was used, so that it can be replaced after its recommended number of uses. The manufacturer of Ventolin has addressed this issue by adding in a counter to the mouthpiece, as has the manufacturer of Advair. Also, depending on the manufacturer, inhalers are sold as a complete unit or the individual canister as a refill prescription.

In the US, Up until December, 2008, inhalers may have contained chlorofluorocarbons as a form of propellant to deliver the medication. Under the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, all inhalers that contain CFCs are being discontinued for hydro-fluoroalkane-pressurized metered dose inhalers (HFA pMDIs).

• Ourasthma.com – a patient-oriented site with information on asthma inhalers.

• Inhaler
• Asthma inhaler