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The question: I took an expired Tylenol for a headache last night and now I'm worried about the potential side effects? Should I be worried?

The answer: It's frustrating when you reach into your medicine cabinet for help relieving your headache, pain or fever, and you're either out of medication or it's expired.

It's not uncommon to accidentally ingest expired medication, but not to fear: In general, most of them are not toxic when expired, but they can lose their effectiveness over time. So, in all likelihood, you haven't done yourself harm – but you may not be getting the pain relief you were looking for.

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Expiry dates are determined and marked on each bottle by the drug maker. When the drug is made, the time it takes to break down (known as its shelf life) is determined. Drug makers guarantee that if their product is used within the shelf life, it will work to its maximum potency and safety.

This being said, even when medication passes its shelf life, it remains partially effective – but quantifying when and how fast the potency decreases is difficult.

An interesting study done by the U.S. military measured how ineffective medications actually become after their expiry date. Because the military carries large drug supplies, they wanted to see how long these could be stored before having to eat the cost of restocking them. The study found that of over 100 drugs tested, over 90 per cent were sufficiently effective several years after their expiry date.

The bottom line: Expiry dates are a conservative measure to make sure you're using medication of the highest potency possible. When you're unwell and all you want to do is feel better quickly, taking something that you know will work is key, instead of worrying whether it's still good or not.

Don't forget that the date listed on the bottle isn't the only way to ensure your drugs remain effective. Proper storage is also an important factor: Keep drugs out of humid rooms and store them in cool, dry places.

If you have a supply of drugs that is old or expired, discard of them safely through your local pharmacy's disposal system or with your doctor's office. Avoid flushing them to avoid potential contamination of the water system.

Dr. Sheila Wijayasinghe is the medical director at the Immigrant Womens' Health Centre, works as a staff physician at St. Michael's Hospital in their Family Practice Unit and at Hassle Free Clinic, and established and runs an on-site clinic at Women's Habitat Shelter in Etobicoke.

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