Part of using an inhaler is knowing when you've used up all the medication. Researchers at Wake Forest University studied what asthma patients do with their inhalers.

What the researchers wanted to know: How do people know when their pressurized metered-dose inhalers are empty?

What they did: The researchers recruited 50 new patients who used inhalers regularly. They asked each one how he or she knew when it was time to get a new inhaler. They also got samples of four different inhalers, and emptied them; they weighed the canisters when they were full, half empty, empty according to the manufacturer's definition, and even emptier. They also tried floating the inhalers at different stages to find out if that was a good way to tell how full they were.

What they found: Thirty-six of the patients (72 percent) said they thought a canister was out of medication when it didn't make a sound anymore when you tried to use it. Four patients said they'd been told if they dropped their inhaler in water, it would float when it was empty, but none of them had ever actually tried it. In their tests, though, floating an inhaler wasn't a reliable way to tell if the medicine was gone. Also, a floating an inhaler often got water on the top of the valve stem, which could keep medicine from escaping the way it's supposed to. Interestingly, while 78 percent of the patients knew they were supposed to shake the inhaler before using it, when they were asked to demonstrate proper use, only half shook the inhaler first. An unshaken inhaler won't deliver the right amount of medication.

What the study means to you: In these tests, all of the canisters still made puffs of air long after the manufacturer said they should be used up. The researchers say the only way to know for sure that your inhaler is finished is to count the number of doses you've used and throw it out when it gets to the number of doses it's supposed to contain.

Caveats: The researchers didn't measure the amount of medication coming out of the inhaler; it could be that the inhaler was delivering plenty of medication even after the manufacturer said it was supposed to be empty.

Find out more: Instructions on using a metered-dose inhaler

Read the article: Rubin, B.K. and L. Durotoye. "How Do Patients Determine That Their Metered-Dose Inhaler Is Empty?" Chest. October 2004, Vol. 126, pp. 1134-1137.

Abstract online: www.chestjournal.org