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Health Canada received nearly 30,000 reports of adverse reactions to drugs and other health products from Canadians last year, a substantial 35-per-cent increase from the previous year.

The department received 27,496 adverse reaction reports last year, compared to 20,360 reports in 2008.

Three-quarters of the reports are considered to be serious, Health Canada said, which can include a "noxious or unintended" reaction that requires hospitalization, causes congenital malformation, leads to persistent or significant disability or incapacity, threatens life or causes death.

While the number of reports may seem high, experts say it likely represent a tiny fraction of actual adverse reactions to drugs and other health products.

"It's certainly the tip of the iceberg," said Tom Perry, clinical assistant professor in the anesthesiology, pharmacology and therapeutics department at the University of British Columbia, and internist at UBC Hospital.

But the report also raises questions about the information it leaves out. For instance, which categories of drugs are most often associated with serious reactions? How many Canadians died as a result of a suspected reaction to a drug last year? And what is Health Canada doing about it?

"Adverse reactions" is a loose term applied to a wide variety of problems relating to health products regulated by Health Canada. In essence, it refers to side effects. In addition to drugs, Health Canada also collects information about side effects associated with natural health products; biotechnology products such as insulin; biologics such as gene therapies; radiopharmaceuticals or radioactive drugs; as well as cells, tissues and organs.

About 70 per cent of 2009 adverse reaction reports were associated with drugs, while 23 per cent were tied to biotechnology products.

However, health officials caution that experiencing an adverse reaction doesn't necessarily mean there is a problem with the drug.

Health Canada would not grant an interview request on the subject, but a spokesman said in an e-mail it's not always possible to prove that a drug caused a particular reaction. The reaction could also have been caused by another factor, such as an underlying condition or a patient's previous medical history, the spokesman said.

The department would not provide any specific information about drugs that are associated with high numbers of adverse reactions.

Dr. Perry said he's encouraged by the fact the number of adverse reaction reports submitted to Health Canada is on the rise. Although the reports don't capture the full scope of drug side effects that occur in Canada each year, they're an important tool in understanding and recognizing potential issues and can potentially help prevent widespread problems before they get out of control.

Pharmaceutical companies are required to tell Health Canada about any reports of serious adverse reactions they receive.

But traditionally, too few doctors and patients have taken the time to report side effects to Health Canada, which means issues with certain drugs can go unrecognized for an extended period, Dr. Perry said.

Although drugs must go through rigorous clinical trials before they're allowed on the market, they are often tested on a relatively small number of people. When a drug is introduced to an entire population, previously undetected side effects or problems can emerge.

The landscape has started to shift in recent years after several major problems with a host of drugs helped put the issue high in the public consciousness. Merck & Co. pulled painkiller Vioxx off the market after evidence showed that it was associated with an increased risk of cardiovascular problems. Debate over the use of the diabetes medication Avandia has been raging for several years after studies showed it is linked to an increased number of heart attacks or episodes of chest pain. Recent reports in The New York Times say some officials in the U.S. Food and Drug Administration want Avandia removed from the market.

Last year, Health Canada also received more than 305,000 foreign reports of adverse reactions, meaning those that occurred outside of Canada. Drug companies are required to submit those reports to Health Canada.