Compressed gas propellants are widely used in pharmaceutical aerosols. The gases generally used include nitrogen, nitrous oxide and carbon dioxide. They can be used to dispense the product in the form of a solid stream, wet spray or foam. The gas is compressed in the container and occupies the head space above the liquid in the can. When the valve is pressed, the pressure of the gas on the contents acts as a piston and pushes the liquid out of the can. The pressure exerted within the container system is dependent on the concentration of the propellant. The amount of gas in the headspace remains the same, but it expands and occupies more volume, and as a result the pressure will drop during the life of the can.

The nature of the formulation and type of the compressed gas used determine the form in which the product will be dispensed, that is solid stream, wet spray or foam. In dispensing of semisolids, a substantially high initial pressure of 90 to 100 psig at 70°F is required to ensure that most of the product is dispensed from the container. The viscosity of the product should be adjusted according to the desired dispensing characteristics. The amount of residual product in the container also varies with the viscosity of the product.

In dispensing of foams, soluble compressed gases such as nitrous oxide and carbon dioxide can be used. The product concentrate is usually in an emulsion form. When the system is used, the gas dissolved in the concentrate will be evolved and causes whipping of the emulsion into foam. To facilitate the formation of foam, the system is shaken prior to use, to disperse some of the gas throughout the product concentrate. For dispensing sprays, with compressed gases as propellants, a mechanical breakup actuator is required. The product is dispensed as a wet spray and is applicable to solutions of medicinal agents in aqueous solvents. Contact lens saline aerosols are formulated with compressed nitrogen gas as propellant and sterilized using gamma radiations.