For the final stretch of their fertilization journey, sperm rev up their whip-like tails to ludicrous egg-boring speed. But amid chemicals from old herbal remedies, sperm may be left feebly treading water a few strokes from the finish line.

Two steroid-like chemicals from plants used in traditional medicines can power-down sperms’ tail engines to prevent the propelling power needed to penetrate their target, Berkeley researchers report in PNAS. The chemicals work by blocking the activation of a calcium ion channel in the sperm, which charge up their tails for final turbo thrusts. Normally, progesterone, a hormone involved in pregnancy that is secreted by cells surrounding eggs, activates this ion channel during sperms’ final approach, the researchers report.

Because the herbal chemicals—called pristimerin and lupeol—are already used in traditional remedies (though not necessarily for anti-fertility purposes), the researchers are hopeful that they’ll easily pass safety tests and offer an effective alternative form of contraception. And, because the chemicals aren’t hormones, unlike other forms of birth control, they may dodge the unpleasant side-effects that arise from messing with complicated and delicate hormone balances in the body, such as changes in mood, weight, and libido. For now, though, the chemicals are just in early lab testing—years away from marketable products even if tests in primates and clinical trials go well.

Lead researcher Polina Lishko, an assistant professor of molecular and cell biology at Berkeley, and her colleagues came up with the two chemicals after working out the mechanics of sperms’ tails. They found that while progesterone could open the ion channel—powering up the tails—other hormones, namely testosterone, estrogen, and hydrocortisone, a stress hormone, blocked the channel from opening in the presence of progesterone.

While looking around for chemicals with similar structures as the hormones, the group came upon pristimerin and lupeol. Pristimerin is from the vine Tripterygium wilfordii, also known as “thunder god vine,” which shows up in traditional Chinese medicines. The vine is typically used to treat arthritis, but it has been linked to anti-fertility effects before. Lupeol, on the other hand, is used for many things. It is found in a variety of fruits and vegetables, including grapes, mangoes, olives, and cabbage, as well as aloe, and is used in traditional medicines all over the world, most notably for anti-cancer treatments.

In lab tests, the two didn’t do much to sperm on their own—the sperm stay alive and motile. But, in the presence of progesterone, the chemicals prevented the ion channel from activating and powering the potent tail flicks. The sperm were left able to gently sway their tails and idly float around. But, they could not build up the speed and power needed to drill down into an egg’s outer layers and fuse to the membrane—which is required for fertilization.

Thus, Lishko and co-authors conclude, the chemicals “serve as promising candidates for contraception, as they reduce the number of hyperactive spermatozoa, thus preventing sperm from reaching and fertilizing an egg.” The researchers are now working on animal experiments to test the potential contraceptive.

PNAS, 2017. DOI: 10.1073/pnas.1700367114 (About DOIs).