ORAL contraceptives may free a woman to have sex without fear of getting pregnant, but they could also extinguish her desire.

The pill has been associated with many side effects, including blood clots, migraines and weight gain. Perhaps least talked about is its tendency to dull libido by decreasing testosterone levels.

Contraceptive drugs curb the hormone's production in the ovaries and also raise levels of sex hormone binding globulin (SHBG), a substance that takes it out of play. But it is unclear how common problems are in pill users. Until now, any sexual dysfunction, including loss of libido, muted or non-existent orgasms or painful intercourse, was thought to be reversible when women stopped taking the drug.

Irwin Goldstein, Claudia Panzer and their colleagues at Boston University studied 125 young women who attended a sexual dysfunction clinic. Sixty-two of them were taking oral contraceptives, 40 had previously taken them and 23 had never taken them. The team measured levels of SHBG in the women every three months for a year, and found that in pill users they were seven times as high as in women who had never taken them. Levels had declined a bit in women who had stopped taking the pill, but remained three to four times as high as in those who had never taken it, the researchers told a meeting of the American Association of Clinical Endocrinologists in Washington DC last week. "There's the possibility it is imprinting a woman for the rest of her life," says Goldstein.