The metered-dose inhaler (MDI) is an aerosol and currently one of the most common types of inhaler. The medication comes out of the inhaler as a mist or spray. A wide range of medications may be delivered by MDI: albuterol / salbutamol, beclomethasone, budesonide, fenoterol, fluticasone, ipratropium bromide, salmeterol, sodium cromoglycate, terbutaline.

Metered dose inhalers (MDIs) are pressurized, hand-held devices that use propellants to deliver doses of medication to the lungs of a patient. These inhaler delivery devices are critically important to public health.

MDIs have unique differences with respect to formulation, container, closure, manufacturing, in-process and final controls and stability.

Metered-dose inhaler products contain therapeutically active ingredients dissolved or suspended in a propellant, a mixture of propellants, or a mixture of solvents, propellants, and / or other excipients in compact pressurized aerosol dispensers. An MDI product may discharge up to several hundred metered doses of one or more drug substances. Depending on the product, each actuation may contain from a few micrograms (mcg) up 34 to milligrams (mg) of the active ingredients delivered in a volume typically between and 100 microliters.

The container, the valve, the actuator, the formulation, any associated accessories (e.g., spacers), and protective packaging collectively constitute the drug product. Unlike most other drug products, the dosing and performance and, therefore, the clinical efficacy of a MDI may be directly dependent on the design of the container and closure system (CCS).

The aerosolization of materials from a pressurized container is a complex and rapid sequence of events. When the content of the metering chamber is released, it undergoes volume expansion and forms a mixture of gas and liquid before being discharged as a jet through the orifice of the actuator.