A metered-dose inhaler

A metered-dose inhaler consists of three major components; the canister, where the formulation resides; the metering valve, which allows a metered quantity of the formulation to be dispenced with each actuation; and an actuator (or mouthpiece) which allows the patient to operate the device and directs the aerosol into the patient's lungs.[1],[2] The formulation itself is made up of the drug, a liquefied gas propellant and, in many cases, stabilising excipients. The actuator contains the mating discharge nozzle and generally includes a dust cap to prevent contamination.

To use the inhaler the patient by presses down on the top of the canister, with their thumb supporting the lower portion of the actuator. Actuation of the device releases a single metered dose of the formulation which contains the medication either dissolved or suspended in the propellant. Breakup of the volatile propellant into droplets, followed by rapid evaporation of these droplets, results in the generation of an aerosol consisting of micrometer-sized medication particles that are then inhaled.[3]


[edit] Uses
Metered-dose inhalers are only one type of inhaler, but they are the most commonly used type. The replacement of chlorofluorocarbons propellants with hydrofluoralkanes resulted in the redesign of metered-dose inhalers in the 1990s. For one variety of beclomethasone inhaler, this redesign resulted in considerably smaller aerosol particles being produced, and led to an increase of potency by a factor of 2.6[4]; however for the salbutamol inhalers the effectiveness and dose required has, by design, remain unchanged.

Asthma inhalers contain a medication that treats the symptoms of asthma.
Dry powder inhalers involve micronised powder often packaged in single dose quantities in blisters or gel capsules containing the powdered medication to be drawn into the lungs by the user's own breath. These systems tend to be more expensive than the MDI, and patients with severely compromised lung function, such as occurs during an asthma attack, may find it difficult to generate enough airflow to get good function from them.
A nicotine inhaler allows cigarette smokers to get nicotine without using tobacco, much like nicotine gum or a nicotine patch. Nicotine inhalers that are marketed as nicotine replacement therapy should not be confused with electronic cigarettes, which produce vapour and which are marketed mainly as devices that smokers can use in non-smoking areas.