Tampons and sex appear to protect women from endometriosis, a painful condition that afflicts about 10 million American women and can cause infertility, according to research by a Yale physician.

The finding came as the result of a study exploring whether sexual behaviors, orgasm, tampon use and douching during menstruation modified the risk of endometriosis. “To our surprise, sexual behavior, orgasm and tampon use during menstruation were found to be less frequent among women with endometriosis compared to controls,” said Harvey J. Kliman, M.D., Ph.D., a research scientist in the Department of Obstetrics and Gynecology and lead author of the study published in the June issue of Gynecological and Obstetric Investigation. “It may be that uterine contractions that are part of the female orgasm induce more effective menstrual-fluid clearance of the uterine cavity, which in turn may facilitate cervical outflow. Further, the use of tampons may be more efficient at the removal of menstrual fluid compared to the use of pads.”

Women with endometriosis have endometrial tissue, which normally grows only in the uterus and is shed during menstruation, growing on the Fallopian tubes, ovaries, other sites in the pelvis or, in rare cases, outside the pelvic area. It is typically found in women who are childless or who have children later in life.

The study has provoked some controversy. “To state that women aren’t getting endo because they’re having sex—when it’s just as likely that they are not having sex because they are experiencing the pain of endo—is jumping to conclusions,” the Endometriosis Association stated on its website. Kliman claims that objections to his conclusions are related to a long-held belief that dioxin in tampons is the real culprit but, he said, “our study refutes this.”

According to Kliman, a backup of menstrual fluid in the pelvic cavity is believed to play a prominent role in the pathogenesis of endometriosis. At the start of his research Kliman held to the conventional wisdom that tampon use, douching and sexual activity, especially with orgasm, at the time of menstruation would heighten the chances of developing endometriosis. Instead, he found that douching did not appear to lessen the risk of endometriosis, but sexual activity, orgasm and tampon use did.

“Our study has an important public health message for women, especially at a time when many women seeking infertility care have endometriosis as their primary diagnosis,” said Kliman. “Our study suggests that tampon use could be one of the strongest protectors against endometriosis.”