Premarin in pill form is one of the oldest estrogen products and most of the data on hormone replacement is based on it.

Premarin, an estrogen product made from the urine of pregnant horses, is already the biggest-selling drug in the United States.

Also emphasized by the "black-box" warning is that estrogen products are not approved for heart disease prevention.

It advises health care professionals to prescribe estrogen products at the lowest dose and for the shortest possible length of time.

But alarming as the findings may seem, their significance is unknown, because researchers do not know which estrogen products the women in the study took.

So, after the risks of Prempro became known, many switched to other estrogen products.

But with 10 million women in this country still taking estrogen products, the agency's decision sends a strong message, medical experts said.

Certain estrogen products may also be used by women after menopause to prevent bone loss (osteoporosis).

These conditions increase your risk of getting blood clots, especially if you are using an estrogen product.

These, in fact, are the only benefits manufacturers are permitted to claim in advertising their estrogen products.