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    Birth Control, HRT, and Sex Drive

    Monkey Study May Help Explain Why Synthetic Hormones Affect Libido
    WebMD Health News

    June 9, 2004 -- A new study examining monkey lust may help explain why so many women taking hormones for birth control or menopause complain of losing their sex drive.

    Researchers from the Yerkes National Primate Research Center in Atlanta reported that female pigtail macaques showed far less interest in sex after being given medroxyprogesterone (MPA), a synthetic form of the hormone progesterone. The study also showed that the monkeys were also more aggressive and anxious when taking MPA than when taking estrogen alone or a combination of estrogen and natural progesterone.

    MPA is the synthetic progestin used in Prempro, the most widely prescribed hormone replacement therapy in the United States, and in the injectable form of the contraceptive Depo-Provera.

    The findings suggest that the impact of MPA on sex hormone-driven behavior differs from natural progesterone, says lead investigator Karen Pazol, PhD.

    "The monkey model is really appropriate for studying (hormonal influences) on sexual behavior," Pazol tells WebMD. "In most species, the ability to engage in sexual behavior is hormone modulated. In primates and humans, ability is in no way controlled by hormones, but hormones do control (sexual) motivation."

    The Study

    The six female macaques were treated with one-week courses of each of the following: estrogen only, estrogen plus natural progesterone, and estrogen plus MPA.

    Pazol says estrogen is commonly given to female macaques to increase sexual motivation.

    In this study, researchers found that adding natural progesterone moderately reduced estrogen's libido-stimulating effect, but adding MPA completely eliminated it.

    The animals exhibited normal aggression patterns while on estrogen or estrogen plus natural progesterone, but aggression levels increased when they were on the estrogen/MPA combination.

    More studies are needed, the researcher say, to determine whether other synthetic progestins, like those used in birth control pills, similarly influence libido and mood.

    "A woman who is susceptible to mood disorders or who is concerned about sex drive may want to talk to her doctor and consider this new information if she is on these therapies," she says.

    Libido Problems Common

    Sexual dysfunction researcher Irwin Goldstein MD, tells WebMD that loss of sexual desire is a common and underreported problem among women taking hormonal contraceptives or hormone replacement therapy.

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