THURSDAY, July 19, 2001 (HealthDayNews) -- If a woman is using a third-generation birth control pill for the first time, her risk of getting blood clots in her veins is almost double that of the average woman using an oral contraceptive, says a new study.
Although the findings, published in the July 21 British Medical Journal, only bolster the general results of previous research, the analysis of five years' worth of research on the Pill and blood clots further defines the risks for women.
Overall, a woman taking a third-generation pill is 1.7 times more likely to have a venous thrombosis than a woman on a second-generation pill; a first-time third generation user is 3.1 times more likely to have a blood clot compared to a second-generation pill user, says the study in the July 21 issue of the British Medical Journal.
Venous thrombosis can in rare cases be deadly, depending on where the clot lodges.
First-generation oral contraceptives essentially contained high doses of estrogen, a female hormone that suppressed ovulation. The second-generation pills, which contained both estrogen and progestogen hormones called levonorgestrel, carried a risk of blood clots, heart attack and stroke.
Scientists creating the third-generation pills replaced levonorgestrel with either of the hormones gestodene or desogestrel, hoping that would cause fewer complications.
But for several years, scientists have been debating the results of studies from late 1995 and early 1996 that suggested a link between third-generation oral contraceptives and the risk of venous thrombosis. Some researchers believed that the findings were interpreted incorrectly because of several potential biases in the studies.
These potential biases included the length of pill use and whether healthy women were prescribed third-generation pills more often than other women.
Algra and his colleagues decided to review 13 studies done on women who used the pill before October 1995, when the British Committee on Safety of Medicines announced there was a link. That announcement was followed by an upsurge in unplanned pregnancies and abortions.
Algra's team reanalyzed the data and found that third-generation oral contraceptives were linked to a 1.7-fold increased risk of venous thrombosis compared to second-generation pills. The risk was highest in first-time users of birth control pills and during the first year of third-generation contraceptive use.
In real-world terms, that means: Although only 3 out of every 10,000 women who take second-generation pills will develop blood clots, 5 in 10,000 women who take third-generation pills will develop them. Although the risk is small, Algra says, it could be reduced even further if women -- particularly first-time users of the pill -- were prescribed a second-generation oral contraceptive.
Furthermore, the researchers found that the biases that had been so vehemently debated in the previous studies were not responsible for the finding of increased risk.
Dr. Alexander Walker, a professor of epidemiology at the Harvard School of Public Health, says that although the newly proven risk isn't dramatic, it's something that doctors and patients should be aware of.
"There aren't any big red flags here, that one of these drugs is a disaster," says Walker. "There's a yellow light for some of them. It's a reason to discuss it, but not a reason to stop taking a pill if you're on it."
Algra says that he would tell a woman concerned about oral contraceptives about this risk. "With regards to venous thrombosis, they [the third-generation pills] are not safer," says Algra. "If this was a woman who has never used the pill, I would say specifically with first-time users that the risk difference is even higher. I would recommend [that she] start with second-generation pills."
And if a woman now taking third-generation pills has had no side effects with second-generation pills, Algra adds, "then I would recommend that she go back to the second-generation pill. If she had side effects with the second-generation pill, you might balance that against the risk increase for venous thrombosis with the third-generation pill. "
Algra and his colleagues also broke down the research findings based on how the studies were funded.
"There's quite a difference between non-industry sponsored studies, which come up with a 2.3-fold increased risk for third-generation pills, as opposed to the industry-sponsored studies, which come up with a 1.3-fold increased risk," he said.
What To Do
You can find information on thrombosis from Boston University's Community Outreach Health Information System, or you can check out the United Kingdom's Committee on Safety of Medicines warning about third-generation oral contraceptives.
