Metered dose inhalers: a need for dose counters

This study investigated the ability of patients to assess when a metered dose inhaler (MDI) is empty.

A total of 17 patients assessed, by shaking, the number of doses remaining in a salbutamol MDI that had been partially emptied. Another 100 patients were asked to collect all salbutamol MDIs they considered to be empty and would have thrown away. These MDI canisters were weighed and the number of doses that had been used was estimated.

Using the 'shaking method', patients overestimated the amount remaining by around 40 doses. The mean number of doses delivered from an MDI labelled to contain 200 doses before it was discarded was 224 (range, 155-289).

These findings confirm that patients are unable to determine when an MDI should be discarded, resulting in insufficient drug delivery at the end of the life of an MDI for the majority of patients, and wastage of the drug for others.