Hormone Drug Helps Fight Hormone-Fueled Breast Cancer, Trial Shows

Key TakeawaysA drug that mimics progesterone might help women fight hormone-fueled breast cancerWomen taking the drug alongside anti-estrogen therapy experienced slower tumor growthProgesterone appears to interfere with estrogen receptors that promote cancer growth.TUESDAY, Jan. 6, 2026 (HealthDay News) — A drug that mimics the female hormone progesterone might help women fight hormone-fueled breast cancer.Women with breast cancer had slower tumor growth if they took megestrol — a synthetic version of progesterone — alongside standard anti-estrogen therapy, researchers reported Jan. 5 in Nature Cancer.“In the two-week window that we looked at, adding a progestin made the anti-estrogen treatment more effective at slowing tumor growth,” co-lead author Dr. Rebecca Burrell of the Cancer Research UK Cambridge Institute said in a news release.Low-dose megestrol – available as a generic — is already prescribed to help patients deal with hot flashes associated with hormone therapy for breast cancer, researchers noted.About three-quarters of breast cancers are ER-positive, which means they feed on the hormone estrogen that naturally circulates in a woman’s body, researchers said in background notes.These women are typically prescribed hormone therapy that lowers their estrogen levels, inhibiting the growth of these cancers. However, this therapy can bring on menopause-like symptoms, researchers said.“On the whole, anti-estrogens are very good treatments compared to some chemotherapies,” senior researcher Dr. Richard Baird of the University of Cambridge said in a news release. "They’re gentler and are well tolerated, so patients often take them for many years."But, he added, "some patients experience side effects that affect their quality of life. If you’re taking something long term, even seemingly relatively minor side effects can have a big impact." To see what impact megestrol might have on a woman’s breast cancer, researchers recruited 198 women with ER-positive breast cancer and randomly assigned them to one of three groups.All of the women were prescribed the hormone therapy drug letrozole. Alongside that, one group took a low 40-milligram dose of megestrol; the second a high 160-mg dose; and the third no megestrol at all.This treatment was given for two weeks prior to surgery to remove the women’s tumors, researchers said.Findings showed that adding megestrol boosted the ability of letrozole to block tumor growth, with roughly the same effect from either the low or high dose.“What was particularly pleasing to see was that even the lower dose had the desired effect,” Burrell said.“Although the higher dose of progesterone is licensed as an anti-cancer treatment, over the long term it can have side effects, including weight gain and high blood pressure,” she said. “But just a quarter of the dose was as effective, and this would come with fewer side effects.”Lab tests using mice showed that progesterone stops ER-positive cancer cells from dividing by indirectly blocking the cancer’s estrogen receptors, researchers said. That results in slower tumor growth.However, follow-up studies are needed to confirm whether megestrol would have the same benefits if prescribed for longer than two weeks, researchers said.More informationThe American Cancer Society has more on hormone therapy for breast cancer.SOURCE: University of Cambridge, news release, Jan. 5, 2026 .What This Means For YouWomen with ER-positive breast cancer should talk with their doctor about whether megestrol might help with hormone therapy..Sign up for our weekly HealthDay newsletter

Hormone Drug Helps Fight Hormone-Fueled Breast Cancer, Trial Shows

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