How does Metered dose inhaler work?
understand your medicine before you inhale.

The pressurized metered dose inhaler was introduced to deliver the asthma medications in a reliable multi dose manner.

 The canister of the metered dose inhaler contains the medicine that is a propellant. Metered dose inhaler consists of a metal canister, which keeps the medication under pressure, and a plastic sleeve, which helps to release the medication. When you press the canister, medicine particles are propelled toward your throat where you can inhale them. MDIs contain a liquid medication delivered as an aerosol spray.

A wide range of medications may be delivered by MDI: albuterol/salbutamol, beclomethasone, budesonide, fenoterol, fluticasone, ipratropium bromide, salmeterol, sodium cromoglycate, terbutaline.
Two common conditions that involve treatment with an MDI include asthma and chronic obstructive pulmonary disease (COPD). The majority of MDIs administer a dose of a bronchodilator, corticosteroid, or a combination of the two, which have an active effect on the lungs.

When you press down on the canister, the actuator pushes a small button on the end of the canister. This releases a specific, measured dose of your medication into the mouthpiece of your inhaler.

As the medication is delivered into the mouthpiece and your mouth, you slowly and deeply inhale a complete breath in so the medication, which is now in aerosol form, will be taken into your lungs as deeply as possible.

Proper technique is critical when using an MDI. If your technique is wrong, too much of the medicine can end up in your mouth instead of your lungs, and it will not have enough medicinal effect. Make sure your doctor or nurse reviews the proper inhaler technique with you at every visit, since it is common for people to get careless with their inhaler technique over time. 

Alternatively, you may be better off using a spacer - a hollow plastic tube between the mouthpiece and the canister of medicine. A spacer makes it easier to get the full dose of medication all the way to your lungs.