Champix is a prescription drug used by smokers trying to quit the habit. (CBC)

Three British Columbia residents are suing Pfizer, alleging the drug company's quit-smoking product Champix can cause serious psychiatric reactions.

Plaintiff Alicia Pickering, 34, said she was a normal, healthy woman until she started taking Champix to kick her smoking habit. Within days of starting the drug, the married mother of two experienced a dramatic change in personality and was consumed with thoughts of dying, she said.

"It literally felt like something had broken in my head," Pickering, of Sechelt, B.C., northwest of Vancouver, told CBC News. "Intense, severe depression overtook me. I would sit on the couch sobbing, not knowing why."

In a writ filed in B.C. Supreme Court, Pickering and two other B.C. residents claim Pfizer "negligently designed, tested, labelled, manufactured and marketed the drug to Canadians." According to the document, Pickering developed bipolar disorder after taking Champix.

The writ also claims the U.S. drug maker failed to warn customers of the adverse side-effects of the medication.

The allegations have not been tested in court.

Company backs product

In July 2009, the U.S. Food and Drug Administration ordered Pfizer to include a warning on Champix, marketed as Chantix in the U.S., to highlight the risk of serious mental health reactions, including changes in behaviour, depressed mood, hostility and suicidal thoughts.

Pfizer says the medication acts on sites in the brain affected by nicotine, helps ease withdrawal symptoms and blocks the effects of nicotine if a user resumes smoking

"Champix is a proven prescription aid to smoking cessation treatment that has helped many adult smokers quit," Pfizer Canada Inc. said in a statement emailed to CBC News. "All medications have potential risks, and patients should consult with their doctors to determine what medications are right for them."

Pfizer added that it would be inappropriate to comment on the specifics of the lawsuit "except to say that the company stands behind the benefits/risk profile of Champix."

More than 1.5 million prescriptions for Champix have been dispensed in Canada since it went on the market in this country in April 2007. Warnings on the monograph accompanying the drug in Canada note reports of serious neuropsychiatric adverse events, Health Canada said.

In January 2009, the department reminded Canadian patients to be on alert for mood and behaviour changes while taking Champix.

Health Canada says that as of Dec. 31, 2009, it was aware of 1,193 cases of suspected adverse reactions involving Champix. The majority of these were psychiatric reactions.

Updating safety information

Health Canada also said it is working with Pfizer to update safety information in the monograph regarding the risk of psychiatric events and to emphasize patients talk to their doctors about the best approach to quitting smoking. The update is expected to be complete by May.

Plaintiffs in the B.C. lawsuit include the mother of Heidi Clow of Victoria, who had taken Champix and killed herself, as well as a woman who says she tried to smash her car into a logging truck after taking Champix.

"We don't know why Pfizer is still selling Champix," said plaintiff lawyer David Klein. "The only explanation is that it makes a good deal of profit from the drug."

Klein would like to see the drug pulled.

Elsewhere in Canada, the death of Thom Jancewicz, a 48-year-old engineer and father, also raised questions about Champix. The Ottawa man killed himself after taking Champix to quit smoking before he started a new job.