Researchers working toward the first daily male birth control pill have announced the results of an experimental trial, calling dimethandrolone undecanoate, or DMAU, safe and effective. While small, the new study found that DMAU was “well-tolerated” in patients, the Chicago Tribune reports.

The study’s findings were reported at a meeting of the Endocrine Society in Chicago. It has yet to receive peer review.

Male Birth Control Pill Is “Safe,” Researchers Say

The drug works similarly to female birth control pills, which have been around for five decades. DMAU relies on a combination of hormones, an androgen and progestin, to reduce a man’s sperm count. As a once-daily tablet, researchers say, the medication could lower hormone levels that are necessary for sperm production, effectively rendering a patient infertile.

Pills In Hand

Lowering Hormone Levels To Knock Out Sperm

In a double-blind study, doctors at two medical centers, one in Washington State and another in California, tested the drug’s safety in 83 men over the course of 28 days. One group of men received a placebo, while the other participants were administered varying doses of DMAU.

“At the highest dose of DMAU tested, 400 mg, subjects showed ‘marked suppression’ of levels of their testosterone and two hormones required for sperm production,” the authors said in a press release for the Endocrine Society.

Mild Side Effects

Previous attempts to develop a male birth control pill have run into problems. Oral testosterone tablets, for example, can lead to severe liver damage and, due to quick metabolization, must be taken twice a day. At this point, DMAU appears to be safer, according to the early test results.

The drug wasn’t associated with any liver impairment; each of the men cleared liver and kidney function tests during the trial.

The only side effects, according to lead author Dr. Stephanie Page, a professor of medicine at the University of Washington, were mild weight gain and a slight reduction in HDL (or “good”) cholesterol. Perhaps even more important, there was no consistent association between DMAU and testosterone imbalances. “Despite having low levels of circulating testosterone,” Dr. Page notes, “very few subjects reported symptoms consistent with testosterone deficiency or excess.”

Once-Daily Pill Would Be “Major Step Forward”

“DMAU is a major step forward in the development of a once-daily ‘male pill’,” Dr. Page says.

“Many men say they would prefer a daily pill as a reversible contraceptive, rather than long-acting injections or topical gels, which are also in development.” Study of an injectable birth control drug for men was cut short in 2016 due to side effects. A surgically-administered gel, Vasalgel, is currently being tested on animals in the United States, with human trials ongoing in India.

DMAU still has a long time before it can hit your local pharmacy. The new study only measured hormone levels, not actual sperm production, so the results are theoretical at this point. Dr. Page says “longer term studies are currently underway to confirm that DMAU taken every day blocks sperm production.”