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A medication highly touted for its ability to help people quit smoking can pose serious health risks - including suicidal tendencies and other psychiatric problems - to those taking it, Health Canada warned yesterday in an announcement that it will be creating a stronger warning label for the drug.

It is the second warning to be issued in recent months about Champix, a smoking-cessation pill sold by Pfizer that has been available on the Canadian market since April, 2007.

Health Canada has received a few hundred reports of psychiatric problems among those taking the drug, including suicidal behaviour, hostility, abnormal dreams or thinking, aggression and depression. Health Canada first issued a warning about the product last June to make the public and health professionals aware of the potential health risks.

Although the department said it is still unclear whether Champix caused the psychiatric problems, new reports of "serious psychiatric adverse events" linked to the drug in Canada and around the world prompted health officials to issue the additional caution. Now, health officials are working with Pfizer to create a stronger warning label for the product, which is expected to be made public in the near future.

In addition to the new reports of side effects, Health Canada said, it issued the warning after Pfizer submitted new information about the drug.

A Pfizer spokesman declined to provide details of any new information pertaining to the drug.

While the drug may pose health problems for some, Health Canada said, the benefits of Champix to the public still outweigh the risks.

"A close monitoring of patients taking Champix by family, caregivers and the patients themselves for adverse events - such as unusual thoughts, feelings or behaviours, especially those related to depression, aggression or self-harm - and immediate discontinuation of the drug should any of these signs be present or worsen while taking the drug - is considered a reasonable risk-mitigation approach," spokesman Paul Duchesne wrote in an e-mail.

One drug-safety expert said the warning probably reflects serious concern among health officials.

If the concerns are borne out, they may eventually lead to the drug's removal from the market, said Alan Cassels, a drug policy researcher at the University of Victoria.

"This is what they do before they pull the drug off the market or require the manufacturer to pull the drug off the market. We've seen lots of cases like this," Mr. Cassels said.

A company may be inclined to pull a drug off the market if it starts seeing numerous lawsuits, Mr. Cassels said. Pfizer is involved in several lawsuits in the U.S., where the drug is marketed as Chantix.

The withdrawal of the antidepressant nefazodone in 2003, for instance, was preceded by a few warnings about the possibility of serious liver problems and several lawsuits.

Health Canada said it will continue monitoring the drug and will update the public if any further action is necessary.

Part of the reason for the new warning is to increase awareness of the potential risks. Even though Health Canada aired concerns about Champix last year, the drug seemed to rise in popularity.

From April to the end of December, 2007, its first year on the market, just under 259,000 Champix prescriptions were dispensed from Canadian retail pharmacies, according to IMS Health Canada, which tracks the pharmaceutical industry. That number jumped to nearly 1.1 million from January to November last year. The prescriptions are worth an estimated total of more than $99-million, IMS Health Canada said.

Pfizer Canada's manager of corporate communications said the new warning is intended to remind Canadians who may be at risk about side effects, and to reiterate that the drug is still considered safe and effective.

"Champix is absolutely a safe product. It's the most efficacious product out there," said Christian Marcoux, adding that people who want to quit smoking should work with a health-care professional.

Mr. Marcoux said it's common for Health Canada to update the safety information of a particular drug, and that the company has confidence in Champix.