Women on St. John's Wort May Become Pregnant Despite Taking the Pill

Taken correctly, the birth control pill is exceptionally effective at preventing pregnancy.

But women who take St. John's wort, an herb often used to treat depression, as well as those who are overweight can become pregnant while on birth control pills.

That's the conclusion of a study presented Sept. 11 at the annual meeting of the Association of Reproductive Health Professionals in La Jolla, Calif.

The new findings lend support to previous research and anecdotal reports about a link between excess weight, St. John's wort and contraceptive failure.

After numerous anecdotal accounts of St. John's wort hampering the Pill's effectiveness, Patricia Murphy, a certified nurse midwife at Columbia University's Center for Complementary and Alternative Medicine, decided to do a more scientific study.

"St. John's wort has been believed to speed up the metabolism of oral contraceptives," Murphy says. If women metabolize them faster than they should, their effectiveness could be compromised.

Three years ago, the U.S. Food and Drug Administration issued a health advisory saying St. John's wort could interact adversely with other medicines, including birth control pills.

In Murphy's study, she evaluated 16 women, aged 20 to 32, who all began taking the same low-dose birth control pill at the start of the four-month investigation. For the first two months, the women took a placebo herb, but did not know if it was fake or St. John's wort. Then their progesterone blood levels were measured. When progesterone reaches a specific level, it suggests you have ovulated and are fertile, Murphy explains.

For the next two months, the women took St. John's wort and their blood levels of progesterone were again evaluated.

"During the placebo [phase], one women had elevated progesterone levels, suggesting she ovulated on the low-dose contraceptive," Murphy says. But three women on the herb had progesterone levels high enough to indicate ovulation had occurred, she found.

More study is needed, Murphy says, but "things were definitely different with the St. John's wort."

Excess weight can also hamper the Pill's effectiveness, according to another study to be presented at the meeting by Dr. Paul Norris, an assistant professor of obstetrics and gynecology at the University of Miami.

Women with a body mass index, or BMI, over 25 are nearly three times more likely to experience birth control failure resulting in pregnancy than leaner women, he found. (BMI is a calculation of weight and height to assess health status; BMIs of 25 and higher are considered overweight. A woman 5-foot-4 who weighs 155 pounds has a BMI of 26.)

And an obese woman (a BMI of 30 or greater) had double the risk of pill failure compared to an overweight woman, Norris found.

Norris tracked 514 pill users who had been patients at the University of Miami's Reproductive Health Clinic from 1994 to 2002. Of the total, seven women had contraceptive failure resulting in pregnancy. Five of those seven had a BMI greater than 25, and three had a BMI in excess of 30.

Norris notes a larger study is needed, but his is not the first to link excess weight with reduced contraceptive effectiveness. Last year, researchers reported in Obstetrics & Gynecology that women taking low-dose pills and weighing 155 pounds or more had a 60 percent higher contraceptive failure rate, possibly because they metabolize the pills differently than lighter women.

To make the pill as effective as possible, experts advise women to take it exactly as instructed. When women do so, fewer than one in 1,000 will become pregnant over a year's time with combination pills -- the type that include both estrogen and progestin, according to Planned Parenthood Federation of America. Five of every 1,000 women who take progestin-only pills become pregnant after a year with perfect use.

More information

For more on oral contraceptives, visit Planned Parenthood and Family Health International.

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