Premarin is the biggest brand name of estrogen -- in fact, it’s long been in the top five best-selling drugs overall in the country for years -- and it's the one that doctors often automatically prescribe when you say you’re ready to go on HRT. On the market since the 1950s, Premarin is the form of estrogen used in the bulk of the research studies on HRT, which means that most things you read about estrogen are actually specifically about Premarin.

Premarin is a mixture of over ten different estrogens -- including estrone (which we make in our own bodies), and equilin and equilenin (horse estrogen -- which, of course, we don’t make in our own bodies). In fact, the name Premarin comes from "pregnant mare’s urine". . . which is its source. Because it comes from horse urine, the pharmaceutical company that makes Premarin (Wyeth-Ayerst Laboratories) considers it a natural estrogen. They are technically right; after all, horse urine is definitely natural! But by most standard definitions of natural as it applies to HRT, Premarin isn’t considered one of the "naturals" since isn’t identical to the estrogen we produce in our own bodies.

When you take conjugated estrogens, your body converts them into active estrogens -- and uses them as it would estrogen your ovaries have produced. About 10 to 15% of the estrogens, however, can’t be used.

Some people have a bias against Premarin because of its source, but, in all fairness, because conjugated estrogen is similar to our own estrogen, Premarin has been shown to eliminate the symptoms of menopause, to prevent bone loss and appears to protect against cardiovascular disease. In addition, studies conducted by the pharmaceutical company indicate that the horse estrogen may be more effective than human estrogen in improving blood cholesterol levels and may actually help prevent the increased risk of breast cancer. (These findings, however, haven’t been corroborated by an outside study, so it’s difficult to be sure about these claims. In fact, other studies hold that Premarin, because it isn’t natural to your body, may cause cancer, while natural estrogens won’t. It’s yet another facet of the ongoing HRT debate.)

On the downside: It appears that you get a higher amount of estrogen in your blood with Premarin than you do with other estrogens. The culprit here is the equilin. The amount of equilin you get from Premarin is much higher than your normal level of the human estrogens, estradiol and estrone. Because it is so strong, it appears to tax your liver more than other non-equine estrogens do, which can be a problem especially if you have a history of liver disease in your family or have had liver disease yourself, smoke, are obese, or have high blood pressure.

In most cases, however, the issue, as to whether or not Premarin is the right choice for you boils down to your desire for a natural (that is, bioidentical) estrogen or one that only approximates the estrogen you naturally make. Studies have shown that Premarin will work to combat osteoporosis and heart disease and eliminate symptoms as well as other forms of estrogen. Moreover, because it has been studied so widely, doctors often feel most comfortable prescribing this.

• Standard dosage: .625 mg -- this is the amount most studied and usually has been shown to be the lowest amount required to protect against osteoporosis and heart disease, although recent studies also indicated that .3 milligram will also eliminate symptoms and prevent osteoporosis. The lower dosages’ effect on heart disease isn’t known yet. Also available in .3, .9, 1.25 and 2.5 mg.
• Pros: Widely studied; most commonly prescribed estrogen; appears effective in relieving symptoms and maintaining bone density
• Cons: Not identical to the estrogen your body makes; some women feel uncomfortable taking an estrogen made from horses’ urine; stronger than many other estrogens, so may cause changes in your liver