Despite a common belief that birth control pills cause weight gain, a new comprehensive review of previous studies finds no evidence it is true.

“There’s evidence that women may switch methods of birth control or stop using the pill because they think it contributes to weight gain,” said Laureen Lopez, Ph.D., a review co-author. “The good news is that this review found no evidence to support a causal association between combination contraceptives and weight gain.”

Birth control pills containing two hormones (estrogen and progestin) are the most well-known form of combination contraceptives; however, women can also be given combination contraceptives by other methods such as skin patches or injections.

Lopez, of the nonprofit organization Family Health International, and colleagues analyzed data from 44 randomized controlled trials that compared a combination hormonal contraceptive to a placebo or to another hormonal contraceptive. All studies included information on change in body weight. While the evidence was not strong enough to rule out the possibility of weight gain with combination hormonal contraceptives, it did not find a major effect.

The review appears in the current issue of The Cochrane Library, a publication of the Cochrane Collaboration, an international nonprofit organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

Only three of the 44 studies compared a hormonal contraceptive to an inactive placebo. Two were trials of contraceptive pills, while one used a skin patch. None of these three studies showed a significant increase in weight in women using the contraceptive compared to those using the placebo.

For the 41 head-to-head studies that compared different hormonal contraceptives or different doses or regimens, most comparisons did not show substantial differences in weight.

“Weight gain is a major problem in the U.S.,” said Lopez. “Over time, many people do put on weight, and some women’s perception about weight gain may be that it’s because of the pill.”

Fear of weight gain is one of the most common reasons that women don’t use - or discontinue use of - the pill. In a national survey of American women ages 18 to 45, published in 2000 in the International Journal of Gynecology and Obstetrics, half believed that oral contraceptives cause weight gain, and one in five women claimed that fear of gaining weight was a reason they would not start or would discontinue the pill.

“The myth that the pill causes weight gain is deeply entrenched,” said Carolyn Westhoff, M.D., professor of obstetrics and gynecology at Columbia University, “and the most weight-obsessed segment of the female population just won’t take hormonal contraception at all because they think they’re going to gain weight.”

According to Westhoff, myths about hormonal contraceptive-related weight gain are shared by many physicians and health educators. “Overwhelmingly, doctors have the same incorrect myths as the general population,” she said, “and sometimes the counseling that patients receive by health educators incorporates and deepens these myths.”

Combination oral contraceptives have been available since 1960 and are the most popular form of reversible birth control in the U.S. Approximately 11.6 million American women use oral contraceptives, including 40 percent of women between the ages of 18 and 24. If used correctly and consistently, they are over 99 percent effective in preventing pregnancy. With typical use, their effectiveness is estimated to be about 92 percent.

Newer combination hormonal contraceptives that have been approved by the Food and Drug Administration (FDA) since 2000 include a skin patch, a vaginal ring and an injectable form of estrogen and progestin. Their effectiveness in preventing pregnancy is similar to that of the pill.

The pill was the most frequently studied form of hormonal contraception in the review. Forty of the trials included an oral contraceptive as one of the comparisons.Only five of the 44 trials in the review included newer non-pill forms of hormonal contraception.

The review was supported by grants from the National Institute of Child Health and Human Development and the U.S. Agency for International Development. The review discloses that one of the authors, Dr. D.A. Grimes, has consulted with or served on a speakers’ bureau for pharmaceutical companies that manufacture hormonal contraceptives and another, Dr. F.M. Helmerhorst, has supervised studies sponsored by or assigned by pharmaceutical companies that manufacture oral contraceptives.

Gallo MF, et al. Combination contraceptives: effects on weight. The Cochrane Database of Systematic Reviews 2006, Issue 1.