This article fails to make the distinction between the synthetic hormone progestin and the naturally occurring hormone progesterone. Progestin is medroxyprogesterone acetate (MPA), which is manufactured from the urine of pregnant mares. MPA is 10 to 100 times more potent in its effects on women than natural progesterone and does not produce the same reactions in the body. The terms progestin and progesterone are often used interchangeably, while in fact the differences between the two are biologically significant. Estrogen without opposition from progesterone is still dangerous for women, and at our clinic, we highly recommend that women use natural estrogen in combination with natural progesterone.

Phyllis Bronson
Aspen Clinic for Preventive and Environmental Medicine
Aspen, Colo.

The letter above, from Phyllis Bronson, contains important errors of fact that should be corrected. First, progestin is not medroxyprogesterone acetate. The term progestin refers to any of several progestational hormones. Second, medroxyprogesterone acetate is sold under the trade name Provera and doesn’t come from equine urine. There exists a clinical preparation of conjugated estrogens that does come from pregnant mares’ urine, Premarin.

Dale E. Hammerschmidt
University of Minnesota
Minneapolis, Minn.

The writer of the original letter, Phyllis Bronson, had caught her error about progestin and mares’ urine. She wrote to Science News that her sentence should have read, “Progestin is medroxyprogesterone acetate (MPA), often prescribed under the trade name Provera, the synthetic progestin.” Unfortunately, her correction didn’t catch up with her original letter before publication .–The editors I find it interesting that so much has been in the media about the increased breast cancer risk of hormone-replacement therapy containing estrogen and progestin, but there’s been no mention of birth control pills, which are very similar. Millions of women use birth control pills, sometimes for 30 years. Why the selective silence or lack of comparison studies?

Carol Harvey
San Anselmo, Calif.

Many research groups have done studies concerning birth control pills and breast cancer. An analysis containing data on more than 150,000 women, which was published in Lancet in 1996, found that there is a small increased risk of having breast cancer diagnosed during pill use and for 10 years thereafter, and no increased risk 10 or more years after the women in the study stopped taking the pill. –N. Seppa