A metered-dose inhaler is designed to deliver a predetermined number of inhalation doses. The number of days before it must be replaced can be estimated from the number of daily doses. Continuing to use the canister beyond the documented number can result in insufficient medication for the patient.



(1) number of metered doses in the inhaler

(2) number of doses per day


Factors affecting the accuracy of the estimate:

(1) accidental discharges

(2) improper use (partial dose, etc.)

(3) amount of excess drug supplied by manufacturer


number of days that inhaler can be used =

= ROUNDDOWN((number of doses in inhaler) / (number of doses per day))


Keeping track of the doses taken is a more reliable measure, but this calculation can help alert the user to have an extra inhaler on hand for when the current use canister is exhausted.


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