Canadian women looking for a trouble-free way to prevent pregnancy may be making themselves susceptible to osteoporosis, a condition that causes bones to become fragile and more likely to fracture.

A $700-million class action lawsuit has been filed against the makers of Depo-Provera, a birth control injection prescribed to more than 600,000 Canadians last year. The lawsuit alleges that women who have taken the contraceptive have developed osteoporosis.

Depo-Provera is made by international drug conglomerate Pfizer, also responsible for popular prescription drugs Viagra, Zoloft and Celebrex. Users are given an injection every three months and have a less than one per cent chance of getting pregnant while on the medication, according to the drug maker's website.

Tracey Bourque, 33, recently found out she has the bone density of a 55-year-old woman. Bourque and her doctor wonder if it is linked to her use of Depo-Provera.

Depo-Provera was tested in the 1980s and approved in Canada in 1997. Two million prescriptions for the drug have been written in Canada in the past four years.

One year ago, Pfizer warned Canadian and American officials the drug could lead to bone density loss. The United States issues a warning shortly afterwards saying the drug should not be used as a long-term contraceptive. Health Canada issued similar statements in November, 2004, and June and July this year.

"The data indicate that women who use Depo-Provera may lose significant (bone mineral density)," states the July Health Canada warning. "The data also indicate that bone loss is greater with increasing duration of use and may not be completely reversible."

It wasn't until she stopped getting her injections two months ago that Bourque learned about the advisory.

"I'm angry and confused," she told CTV News. "What is my future now?"

Bourque has now been prescribed the drug Actonel -- usually given to women post-menopause who have significant bone loss in a bid to rebuild it. She will also be taking calcium and vitamins in a bid to counter her unusual bone density loss.

According to lawyer Glyn Hotz, Bourque isn't the only concerned person. He told CTV that he has filed a class action lawsuit on behalf of a group of woman who also worry about a link between Depo-Provera and bone deterioration.

"We've seen people who have had multiple fractures, required knee replacements," he said Monday. "We've got somebody who is in her twenties who is on the borderline for having a hip replacement."

The lawsuit is one of three that has been filed in Canada.

Pfizer Canada responded to the claims in a press release, saying it plans to "vigorously defend" the lawsuit. (See the end of this article for the full statement.) It says the suit is "without merit" because the drug has been "safely used by millions of women around the world for decades" and that "all medicines have benefits and risks."

Tina Cross, who founded an Internet support group for present and past users of the drug, doesn't think enough woman have heard about the controversy.

"I feel that there's thousands of women who are simply not aware," Cross told CTV. "I would strongly recommend that they follow up with their doctor, certainly if they have been on this medication longer than two years."

PFIZER CANADA STATEMENT REGARDING DEPO-PROVERA CLASS ACTION

Pfizer Canada said today it plans to vigorously defend a class action lawsuit filed against the company in Ontario Superior Court in relation to Depo-Provera (medroxyprogesterone acetate suspension for injection).

Pfizer Canada believes the action is without merit. Depo-Provera is an important treatment option for Canadian physicians and patients and has been used safely by millions of women around the world for decades.

All medicines have benefits and risks. It is important that patients discuss with their physicians and understand the benefits and risks before taking any medication. Since the approval of Depo-Provera, as additional clinical and safety information has become known, Pfizer has worked collaboratively with Health Canada to determine appropriate actions, including updating the product monograph and prescribing information, distributing letters to healthcare professionals, and issuing public advisories, to ensure that Canadian physicians and patients have the information required to make appropriate treatment decisions.

Pfizer is committed to the safety of Canadian patients and works closely with Health Canada to ensure that the most up-to-date safety information for all of our products is included in the product monographs and prescribing information and that this information is communicated to physicians and patients.