Women who use over-the-counter progesterone creams could be exposed to hormone levels comparable to those used in a form of Hormone replacement therapy (HRT), researchers said on Wednesday..

The study compared one brand of progesterone cream with a prescription oral progesterone that, like the cream, is derived from plant sources and provides a hormone that is structurally identical to the natural progesterone in a woman’s body. The prescription product, Prometrium, is used as part of Hormone replacement therapy (HRT) after Menopause, and for restoring menstruation in younger women whose periods have stopped because of another illness.

The study found that the over-the-counter cream, when taken as the label directs, exposed study volunteers to as much hormone as the prescription did.

The potential problem, researchers say, stems from the fact that these creams are categorized as herbal beauty products and women use them without medical supervision - for a range of conditions the products purportedly aid, from Premenstrual syndrome (PMS) to hot flashes to Osteoporosis.

And the imprecise nature of using a cream means there is a high probability of taking more than the intended dose.

“In real life, people tend to slather things on,” said Dr. Joseph S. Bertino, Jr., of Bassett Healthcare in Cooperstown, New York.

Bertino, the senior author on the study, stressed in an interview that the research did not look at the potential health risks of natural progesterones but only the body’s absorption of them.

Because natural progesterone creams are not considered drugs, they do not have to be studied for safety and effectiveness before going on the market.

For its part, Prometrium may increase the risk of blood clots, while more common side effects include headache, dizziness, abdominal cramps and breast tenderness. In addition, the relationship between natural progesterone and breast cancer is not clear, and it’s recommended that women with the disease not take Prometrium.

If over-the-counter creams expose women to similar levels of progesterone as pills do, their effects in the body could be comparable as well, Bertino and his colleagues note in their report in the Journal of Clinical Pharmacology.

For the study, the researchers had 12 healthy postmenopausal women each use a single brand of over-the-counter progesterone cream for 12 days; a “calibrated teaspoon” was used to ensure that they applied the cream according to the product’s directions. Each woman also used the oral progesterone for 12 days, with at least a month between treatments with the two products.

Blood tests taken on the last day of each treatment period showed that, even with judicious use, the cream exposed the women to as much progesterone as the oral prescription did. Side effects, namely headache, were also similar with each product.

Because the skin slowly absorbs medication, Bertino explained, the cream led to steady concentrations of progesterone in the women’s blood throughout the day, as opposed to the “peaks” and “troughs” associated with the oral hormone.

But the women’s overall progesterone exposure was the same with both products, he said.

Given that - and the fact that women may actually use much more of the product than is recommended - the safest move, according to Bertino, is to consult a doctor about using the creams.

SOURCE: Journal of Clinical Pharmacology, June 2005.