All of the monkeys were then exposed vaginally to SIV, a virus that causes an AIDS-like disease in monkeys. Only one of the 10 monkeys in the placebo group was infected. In an interview, Marx said the study demonstrates that the thinning of the vaginal wall by progesterone increases the ability of the virus to cause infection in monkeys. The vaginal lining is a strong barrier against infection. Progesterone will thin that barrier.
Progesterone is a natural hormone produced by the ovaries.
Two contraceptives marketed in the United States, Depo-Provera and Norplant, contain synthetic hormones called progestins that mimic many of the effects of progesterone. About 1.5 million American women use Depo-Provera and about 1 million use Norplant. Oral contraceptives, which are the type most widely used in the United States, do not have the same effect on the vaginal lining as the injected contraceptives, said Marx. The formula of the pills includes estrogen, which tends to thicken the lining, and he said this may offset the thinning effects of the progesterone. Both companies said Marx's study emphasizes the need for women to practice safe sex by using a condom and that contraceptive drugs do not protect against sexually transmitted diseases.