Birth Control, HRT, and Sex Drive
Monkey Study May Help Explain Why Synthetic Hormones Affect Libido
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 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.
"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