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.
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 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.
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.
"The message I take from this study and my own experience is that you shouldn't play with Mother Nature," he says. "The synthetic hormones don't act in the exact manner of natural progesterone, so it is not surprising that they affect women differently."
But gynecologist and best-selling author Judith Reichman, MD, who wrote a book about sexual desire problems in women, says there is no clear evidence that women taking menopausal hormone therapy have fewer libido problems when they use natural hormone preparations. All hormonal birth control preparations contain synthetic hormones.
"There is more evidence that the way the hormone is delivered is important," she says. "Oral delivery has more of an effect on libido than transdermal or transvaginal delivery."
Goldstein cited a recent study showing fewer libido problems among women taking a birth control pill that delivers three progestin strengths instead of two, called triphasic birth control pills.
"If a woman on oral contraceptives complains of sexual desire problems, I might switch her to a triphasic pill, and if that doesn't work I might switch her to another form of birth control," she says. "Some women do fine on oral contraceptives and others completely lose their sex drive."