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I've unwittingly been taking pain medication that expired last year. Is this dangerous?

Most likely not. But you're probably feeling more pain than you should. The expiration date on any medication is the point at which the active ingredient has lost 10 per cent of its effectiveness — and efficacy continues to wane as weeks and months pass.

Whether this is dangerous depends on the medication. In some cases, as the active chemical breaks down, other ingredients can change into new agents that can be dangerous, but studies have yet to confirm this, says Maureen Donovan, PhD, a professor of pharmaceutics at the University of Iowa.

Her advice: Give your expired meds a sniff test, as some drugs smell different when they go bad. Aspirin, for example, reacts with moisture in the air to create acetic acid (vinegar). And when gelatin casing on gelcaps starts to oxidise, it releases a stomach-turning odour you'd be crazy to ignore.

To keep your meds at full strength, Donovan advises against storing them in the medicine cabinet in your steamy bathroom — or in your car. "Store them in a dry, dark place, like a hall closet," she says.