To use a metered-dose inhaler, the person shakes the inhaler and removes the cap. The person exhales completely, then brings the inhaler close to or in her mouth and breathes in slowly while pressing the top of the inhaler. She continues to inhale as a puff of medicine sprays out. The person then holds her breath and subsequently exhales again. Exhaling fully helps clear air from the lungs, so that more air can be inspired with the next inhalation. The more air is inhaled, the more of the drug can also be inhaled and thus reach the more distant, smaller airways.
(Video from Lahey Clinic Media Center.)