A growing number of U.S. women are filling prescriptions for unregulated hormones to ease menopause symptoms such as hot flashes and night sweats, a recent study suggests.

Researchers focused on what’s known as compounded hormone therapy – prescriptions that are custom-blended by pharmacists instead of factory-made by drug companies and approved for sale by the U.S. Food and Drug Administration (FDA).

Sometimes these formulations are marketed as “bioidentical,” and touted as more like naturally occurring hormones than regular pharmaceuticals.

Based on survey data from almost 500 pharmacists, the new study estimates that the number of prescriptions for unregulated compounded hormone therapy has reached an estimated 26 to 33 million a year, almost equal to the 36 million annual prescriptions filled for FDA-approved treatments.

“Despite the potential increased quality risks and the lack of safety and efficacy data for non-FDA approved or FDA regulated compounded bioidentical hormones, their use by menopausal women was found to be higher than expected and the pharmacists who participated in this survey believe it will continue to grow,” said study co-author Dr. JoAnn Pinkerton, executive director of the North American Menopause Society.

“If women and their providers decide to use compounded hormone therapy, education should be provided about the unique risks of compounding,” Pinkerton, also a researcher at the University of Virginia Health Center in Charlottesville, said by email.

Many women have been reluctant to use FDA-approved hormone replacement therapy (HRT) to ease menopause symptoms since 2002, when the federally funded Women’s Health Initiative (WHI) study linked the treatments containing man-made versions of the female hormones estrogen and progestin to an increased risk of breast cancer, heart attacks and strokes.

Compounding drugs are not FDA-approved, though U.S. regulators may inspect pharmacies to identify problems such as unsterile conditions that might lead to the distribution of contaminated medicines. Doctors may prescribe compounded drugs to give patients an option that isn’t commercially available, such a dye-free medicine for people allergic to the colorings or a liquid or suppository version of a drug that isn’t normally sold that way.

Compounded hormone therapy requires a prescription, and may contain the same active ingredients – synthetic estrogen and progestogens – that are in the mass-produced versions. Pharmacists may mix in different inactive ingredients and offer different doses than the FDA-approved drugs.

The WHI study raised concerns about Prempro and Premarin, both drugs made by Wyeth, which was acquired by Pfizer Inc. in 2009.

Many women turned instead to compounded hormone treatments, incorrectly thinking they might avoid risks associated with the FDA-approved pills, Pinkerton said.

For the current study, Pinkerton and co-author Dr. Ginger Constantine, head of women’s health at Wyeth when concerns about Prempro and Premarin emerged, analyzed survey data from pharmacists at 365 independent community drugstores and 118 compounding pharmacies.

Based on the survey responses, the researchers estimated that annual sales of compounded hormones have reached $1.3 billion to $1.6 billion, and may grow as much as 5 percent to 25 percent over the next two years.

It’s possible that the pharmacists who agreed to participate in the survey weren’t representative of all independent drugstores and compounding pharmacies in the U.S., the researchers acknowledge in the journal Menopause.

Even so, widespread compounded hormone use underscores the need to educate women about their treatment options, said Dr. Cynthia Stuenkel, a researcher at the University of California, San Diego School of Medicine who wasn’t involved in the study.

“There is no scientific evidence to support claims that compounded hormone therapy is any safer or even as safe as FDA-approved therapies,” Stuenkel said by email.

Women who get compounded hormones may also be exposed to the same risks tied to FDA-approved treatments, and they may face additional risks such as the possibility that the version mixed by pharmacists might have an incorrect dose or be contaminated with unsafe ingredients, noted Dr. Nanette Santoro, a researcher at the University of Colorado School of Medicine who wasn’t involved in the study.

While compounded hormones might make sense for some women who are allergic to ingredients in the FDA-approved drugs, most women don’t need custom-blended therapies, Santoro said by email.

“I generally recommend against using compounded hormones,” Santoro said.