Many inhalers have dose counters, telling you how many doses are left. Lots don’t though, particularly relievers (usually blue) and steroid inhalers (usually brown). You may have heard, or come to the conclusion yourself, that you can tell if your inhaler is empty by shaking it or seeing if the spray still comes out. Forgive me if I seem paternalistic but you probably can’t! Metered dose inhalers (spray inhalers) are designed to deliver a set number of doses of medication. After that you will still get a visible spray of propellant but you won’t be getting as much of the active ingredient. Doses “tail off” in an unpredictable way.
There is really only one reliable way to tell if your inhaler is empty. That is to know the number of doses your inhaler contains, and keep a track of how many doses you’ve taken. You could make an educated guess, like my inhaler has 200 doses in and I use 4 doses a day, so my inhaler will last for 50 days.
There is one other trick, that you might want to use, but bare in mind it ONLY WORKS FOR VENTOLIN EVOHALER. It will not work for other sorts of blue inhaler like Salamol or AirSalb. These clever chaps in Switzerland had the idea that you could weigh your inhaler to see how full or empty it is. They found that it worked very well, and validated their results on highly accurate laboratory scales, and on the Ikea Ordnung kitchen scale. Interestingly the cheap kitchen scale worked very well. The table below shows how to interpret the weight. You have to remove the canister from the plastic case by pulling it out in order for the weights to be accurate. Did I mention – it ONLY WORKS FOR VENTOLIN EVOHALER?!
We’ve found this technique very helpful on repspiratory wards where 12% of patients were unknowingly using empty inahalers. We don’t recycle any ventolin inhalers on our respiratory ward without weighing them first to confirm they’re empty.