TORONTO - Shortages of various prescription drugs are occurring more frequently, but doctors and patients have no way of knowing when a medication will be unavailable because pharmaceutical companies aren't required to publicly report gaps in supply.

Last fall, following a request by federal Health Minister Leona Aglukkaq, some Canadian drug companies began voluntarily reporting shortages of certain drugs on public websites.

But a growing number of doctors, patient advocacy groups and health organizations say government should make it mandatory for pharmaceutical companies to warn of impending drug shortages so that prescribing physicians, pharmacists and patients aren't caught by surprise.

Jeff Morrison, director of government relations for the Canadian Pharmacists Association, says there have been sporadic episodes of drug shortages for many years -- but they are becoming more frequent.

"What's changed is that for the past year and half or so, the intensity and the number of shortages that are occurring and the length of the shortage has really been almost unprecedented," Morrison said Tuesday from Ottawa.

Medications periodically in short supply include some used to treat various types of cancer, chemotherapy-related nausea and epilepsy. Even penicillin has occasionally gone missing from pharmacy shelves, he said.

Morrison said a survey of the group's members in 2010 found that some pharmacists were spending at least half an hour a day trying to deal with prescriptions they were unable to fill because of medication supply gaps.

That often meant calling other local pharmacies and even hospitals looking for the product or contacting the prescribing doctor to determine an alternative medication for their patient.

"And when they have to say, 'Listen, the drug you've just got prescribed, we don't have it,' it's not easy for them," said Morrison.

While shortages are frustrating for physicians and pharmacists, "in some cases, it's the patients that are absolutely suffering."

"For a lot of patients, especially those on long-term medications, they don't trust alternatives, they're not interested in Plan Bs or Plan Cs. They want to know that the drug that they're used to is still available. And when it's not, sometimes they don't take it or they'll take it improperly.

"You can't develop an alternative-care program when you don't know when a certain product is available or not," added Morrison, whose association has been working with manufacturers to set up online databases of drugs in short supply.

The databases -- http://druginfo.usask.ca/healthcare--professional/canadian--drug--short ages.php and http://www.fridaypm.ca/ -- include a number of products from different drug companies, but it's not known how complete the listings are.

Information about unavailable brand-name drugs also can be found at: www.canadapharma.org/shortage/index.asp?l=en.

Meanwhile, Health Canada is encouraging the pharmaceutical industry and professional health-care associations to continue to collaborate to establish a national, one-stop drug shortages monitoring and reporting system.

"Health Canada will also continue to look to industry to work co-operatively to prevent drug shortages by addressing their root causes," a spokeswoman said by email.

Dr. Jacalyn Duffin, a hematologist and medical historian at Queen's University, said there are any number of reasons driving the periodic paucity of some medications. And the problem is not unique to Canada: the United States is also grappling with gaps in supply, as are other countries worldwide.

One reason often cited is an inability to access raw ingredients -- many of which are produced in countries such as China and India -- amid a growing global demand for drugs.

"Lots of different causes have been mooted. I think somebody knows the answer and is not telling us," Duffin said from Kingston, Ont.

But she noted that many of the "missing" drugs are inexpensive generics, and there is widespread speculation that pharmaceutical manufacturers may be choosing to reduce or even discontinue production of less profitable generics in order to boost sales of newer, more expensive brand-name drugs.

"I think there's more money to be made by selling expensive drugs, so what you want to do is try to kill the cheap ones, your competitors," she suggested.

Still, Duffin believes drug companies have an ethical duty to make sure Canadians have access to prescribed medications, many of which can be life-saving.

"Making drugs is about people's lives and their health. It's not like making running shoes," she said. "And this duty to supply should be something that they should all take to heart."

Duffin, who has created a website to address the issue at www.canadadrugshortage.com, argues that providing timely information about current or expected medication shortfalls shouldn't be voluntary on the part of pharmaceutical companies, but mandatory.

"I think somehow they should be told that if they want to license and market in Canada, they must give us six months' notice about shortages.

"And yet somehow we're not insisting on that in any way."