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Hormonal birth control for men is one step closer to becoming a reality.

A global clinical trial to test the effectiveness of hormonal contraceptive shots for men found that these shots successfully suppressed sperm production in nearly 96 percent of men who completed the study. When the trial concluded, more than 75 percent of the participants said they’d be willing to continue receiving the shots as their main method of contraception.

But because the participants reported a high number of (mostly mild) adverse reactions to the shots, especially from a single study center in Indonesia, scientists may need to re-formulate the injections or experiment with different injection protocol before testing it again, said study author Dr. Mario Philip Reyes Festin of the World Health Organization in a statement about the research.

“More research is needed to advance this concept to the point that it can be made widely available to men as a method of contraception,” Festin said. “Although the injections were effective in reducing the rate of pregnancy, the combination of hormones needs to be studied more to consider a good balance between efficacy and safety.”

Currently, men’s only options for contraception they can control directly are withdrawal, condoms and a vasectomy. Researchers have been experimenting with a reversible hormonal contraceptive for men for four decades, Festin notes in his study, but no safe, reliable product has ever been developed.

While this particular injection may need a few more kinks worked out, it’s clear that the need for male hormonal birth control is great. A 1999 survey of about 1,300 men in South Africa, China and Scotland revealed that the majority would welcome a new hormonal contraceptive for men, but that a pill would be more acceptable to them than an injection. A corresponding survey among 1,900 women in the same countries found that almost two-thirds of those surveyed thought that the responsibility for contraception fell too much on women. Most of them also said that they would trust their male partners to take a hormonal contraceptive like a ”male pill.”

The trial’s results: impressive, and perhaps culturally influenced

The trial recruited 320 healthy men in Australia, Germany, the United Kingdom, Chile, India, Indonesia and Italy between the ages of 18 and 45 years old who were in stable, long-term monogamous relationships with healthy women. The couples did not want to become pregnant during the next two years that the trial would run, but were willing to accept a low but unknown risk of pregnancy by being a part of the study.

Researchers first checked to see if the men had normal sperm levels, and then suppressed those counts with a hormone injection every eight weeks over the course of 26 weeks.

Once the men hit a certain low sperm concentration level, the couples were able to stop the backup birth control and use the injection as their primary method. Every eight weeks, the participant received contraception shots for a maximum of seven times, or for a total of about one year. At this point, the couples agreed to rely only on the injections for birth control. After the trial was over, the men entered a recovery phase, where doctors observed them to make sure their sperm numbers returned to normal levels.

Of the 320 male participants initially recruited for the study, 274 achieved sperm suppression that was low enough to rely on as birth control. Of those, 266 stayed involved in the study, using the injection as their birth control. By the end of the study, four of them impregnated their partners despite achieving sperm suppression ― an efficacy rate of about 2.2 pregnancies per 100 women who use it for one year. In contrast, careful condom users have a pregnancy rate of about three to five pregnancies per 100 women who use it for one year.

Men reported side effects like depression, other emotional disorders, acne, muscle pain, injection site pain and an increased libido, but interestingly, the bulk of side effects regarding emotional disorders, injection site and muscle pain and increased libido came from a single study center in Indonesia, suggesting that cultural influences may have played a role in how participants perceived their reactions to the shots, explained Dr. Cynthia Harper, a professor of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco. In contrast, participants from India reported very few adverse reactions in general. Reports of acne were distributed among all the centers around the world.

After the study was over, most of the participants regained their normal sperm numbers, although eight men took more than one year to recover. Two of those men declined further follow up, but five of those eight fully regained normal levels after 74 weeks. One participant did not bounce back to normal levels after four years since his last injection. In contrast, female hormonal contraception like the pill takes about one week to start working before couples can rely on it exclusively for birth control, and return to fertility takes just one to three months.

How male birth control could change the contraceptive landscape

The male contraceptive shot didn’t have the same level of effectiveness as the female contraceptive shot or other forms of female hormonal birth control. Depo-provera, which is a hormonal contraceptive shot for women, has an efficacy rate of 0.3 pregnancies per 100 women who use it for one year.

But if this combination of hormones, or another comparable product eventually passed clinical trials and made it to market, Harper predicts a sea-change in how the U.S. views and treats contraception. Currently, hormonal birth control is tied down with cultural baggage about a woman’s role in the world and the politics of abortion, making it a target for religious companies who don’t want to cover birth control for their employees, or politicians who say insurance companies shouldn’t cover birth control at all.

But male options for hormonal birth control could expand cultural acceptance of birth control nationwide, said Harper, leading to universal support for health insurance coverage of any method.

“Coverage has been so hard to come by and is so hotly contested as a policy issue, and I wonder if this would make it less of a controversial policy if there were a male method,” said Harper.

For individual couples, having more options also means that women who don’t respond well to a hormonal birth control could rely on their partner to handle the medication, all while having the same level of confidence that the effectiveness will remain the same.

“A lot of the reason why women don’t use effective contraception in this country is because they’re worried about hormones, but they may be with a partner who is fine with hormones and might happily use a hormonal method,” she said. “It gives this wonderful opportunity for more choice and for individual variation.”