TORONTO -- You'll wash the windows, rake the yard. You may sort through closets and drawers to identify unneeded clothing to give away.

But when you are doing your spring cleaning, do you remember to spend some time clearing out the medicine cabinet? Chances are, if you pop open the mirrored doors of many medicine cabinets, you are likely to see some "just in case" drugs that medical experts wish weren't there.

Didn't take all the Tylenol 3s or Percocets your doctor prescribed after your surgery a couple of years ago? You probably hung on to them just in case they might come in handy later. Ditto the leftover pills from the prescription of antibiotics you stopped taking once you started to feel better. (Stop doing that, by the way. Right now.)

Leftover drugs stored for a rainy day in household medicine cabinets pose a number of risks, two doctors write in a commentary published Monday in the Canadian Medical Association Journal.

The doctors, from Toronto-area hospitals, suggest Canadian households should take advantage of the upcoming Prescription Drug Drop-Off Day -- on May 10 -- to get rid of unused prescription drugs.

Dr. Peter Wu, chief medical resident at Toronto General Hospital, and Dr. David Juurlink, a specialist in internal medicine and clinical pharmacology at Sunnybrook Health Sciences Centre, say inertia and a natural inclination to save something that might be useful later are likely the reasons why people hang on to drugs they haven't needed for the illness for which they were prescribed.

Wu says last year two tonnes of unused medications were turned in to pharmacies across Canada during the first national drop-off day.

There's good reason to think the problem hasn't gone away. A study of people who underwent dermatologic surgery showed 86 per cent of patients didn't use all the drugs they were prescribed and half of them planned to keep the remaining pills.

A systematic review -- a study that reviews accumulated studies on a topic -- found that more than a third of patients didn't take all the prescribed pills when they were put on antibiotics. (Seriously, stop doing that. It's contributing to the development of drug-resistant bacteria, a problem the World Health Organization calls a major threat to global public health.)

The same study showed more than a quarter of patients used leftover antibiotics for subsequent infections -- in other words, they self-prescribed drugs without knowing what they were suffering from, whether it was caused by a bacterium (the bugs that antibiotics treat) or whether that particular antibiotic was useful for the ailment they had.

That's a bad idea, the authors say, for a variety of reasons. Included among them is the fact that self-medicating like this might delay sick people from getting assessed by a physician, it might affect their test results, thereby delaying diagnosis and the start of care.

There are other reasons why keeping leftover drugs around is a really bad idea, Wu and Juurlink say.

Prescription medications in the household can be taken by young children and lead to accidental poisonings. From 2001 to 2008, more than 450,000 cases of poisonings in children were reported to U.S. poison control centres and 95 per cent of those cases involved ingestion of prescription medication, they say.

Drugs in the home can also be misused by someone looking for a high, the authors caution.

"It really is a concern because in many cases we're talking about powerful, potentially dangerous drugs that are just kind of sitting there in the medicine cabinet," says Robert Mann, senior scientist at the Centre for Addiction and Mental Health in Toronto and principal investigator for the Ontario Student Drug Use and Health Survey.

That ongoing survey of students in Grades 7 through 12 reported last year that 15 per cent of high school students in Ontario admitted to using prescription drugs -- often opioids and stimulants -- for recreational purposes. The most common source of the drugs was the family medicine cabinet.

Mann, who was not involved in the commentary written for the medical journal, says using leftover drugs "is not a good idea. Because you don't really know what you're doing and don't know what you're treating."

"This is a very dangerous thing to do. For example, with the prescription opioid drugs, these are powerful, addictive drugs that should only be used under medical supervision."

So what should one do with unneeded drugs? Health Canada recommends turning them in to pharmacies -- the plan for the drop-off day. Another option is municipal waste disposal centres. The department does not recommend flushing unused drugs down the toilet or putting them in the garbage.

Wu says the idea of an annual national disposal day is a good idea, one that may focus people's attention on the problem. But a better idea is to get rid of unused drugs as soon as you determine you don't need them.