In the study, 320 men in long-term relationships received shots of the hormones testosterone and progestogen, which worked to lower their sperm counts to a pregnancy-preventing level.
“When you give yourself synthetic testosterone, your body is going to think that it already has enough of it, so it’s going to shut down your natural production of testosterone, ” says Dr Jamin Brahmbhatt, a urologist at Orlando Health.
As a result, your sperm count plummets. The shot was successful in 96 per cent of the men, the study reports.
The guys got the shots every 8 weeks for up to 56 weeks. During that time, only 4 pregnancies occurred - a failure rate that is similar to what women typically see on the pill, the researchers say.
Sounds pretty great, right? Not so fast.
After reviewing the study, an external committee recommended that the shots stop, and the research came to a halt.
That’s because the men had a high likelihood of worrisome side effects, including mood changes and depression.
In fact, 17 per cent of the guys reported some kind emotional disorder, 5 per cent experienced mood swings, 4 per cent developed depression, and 2 per cent became hostile.
One subject even intentionally overdosed on acetaminophen, which the researchers think might be related to the shots.
Too much testosterone is probably to blame, says Dr. Brahmbhatt. He sees mood changes like aggression in his patients who take testosterone therapy for low T.
But because the guys in the study already had normal testosterone levels, the influx of extra T may have brought them overboard, increasing their risk for side effects even more, he says.
Other side effects, like acne (in 46 per cent of men), injection site pain (in 23 per cent of men), and an increase in libido (38 per cent of men) were common, too.
So what does this mean for the future of hormonal birth control for men?
Basically, the study shows that it works, and guys - and their partners - want it as an option. At the end of the study, 82 per cent of men and 76 per cent of their female partners said they would use it again.
But there are some important kinks that need to be worked out before it can become a viable choice for the public.
More research needs to be done - whether it’s on the dosage of hormones or the timing of them - to figure out a formula that can still lower sperm count while avoiding the serious side effects.
We’re probably 5 to 10 years away from it hitting the market, Dr. Brahmbhatt says.
Other forms of male birth control, like Vasalgel - a procedure that is similar to a vasectomy but is more reversible - may be available to consumers sooner, though that timeline isn’t set in stone yet, either.
So in the meantime, stick to the tried-and-true forms of birth control: With perfect use, failure rates are just 2 per cent for condoms, .3 per cent for female birth control pills, and .1 per cent for vasectomies, according to estimates in the journal Contraception.