Premarin (or PMU) is a very common HRT drug that is derived from pregnant mares’ urine. In fact, its very name stands for PREgnant MAres uRINe (via ScienceDirect).
This method of HRT production began in the 1940s, when it was discovered that estrogen taken from the urine of pregnant mares could relieve menopausal symptoms in women. The resulting industry spread rapidly throughout the northern United States and Canada, says Horsetalk in New Zealand. Today, more than 9,000,000 women take the drug to relieve menopausal symptoms that include vaginal dryness, hot flashes, and night sweats. Premarin may also prevent osteoporosis.
According to Forum Health, Premarin contains many types of equine estrogens, and only two are bio-identical to human estrogens. The others are much stronger and last longer than human estrogens, and they are suspected of having more side effects in humans.
Not surprisingly, the practice of keeping pregnant mares in small stalls for most of their one-after-another pregnancies, attached to urine bladder bags to collect all their urine, has raised the ire of animal welfare agencies. As a result, most PMU farms are now located in Canada, says Horsetalk, and overseas (via Gentle Giants Draft Horse Rescue).