Researchers say they have developed an injectable male contraceptive that may prevent pregnancy, but there are some worrisome side effects.

Fifty years ago men had three choices when it came to contraception: condoms, the “rhythm method,” and vasectomy.

Today, men still are limited to the same three choices.

Now, the announcement of a potentially new injectable male contraceptive may change all that.

A study, published last week in the Journal of Clinical Endocrinology & Metabolism (JCEM), explains how hormonal shots could prevent pregnancy in the female partners of men who get the shot.

For the study, intramuscular injections of 200 milligrams of norethisterone enanthate combined with 1,000 milligrams of testosterone undecanoate were administered every eight weeks. There were 10 study centers involving 320 men aged 18 to 45.

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The shots eliminated the need to use condoms or female birth control before engaging in sex. But there were problems.

After 52 weeks, the cumulative reversibility of sperm production was about 95 per 100 continuing users.

That means that four weeks after leaving the test groups, five of every 100 men were still unable to produce sperm at their previous levels. It is not known if their sperm production increased after time.

Other adverse effects included acne, injection site pain, increased libido, and mood disorders.

As a result, following the recommendation of an external safety review committee, the recruitment and hormone injections were terminated early.

One test subject experienced depression. Another had an irregular heartbeat, which concerned the review committee enough to find it unwise to continue.

Still, as far as its primary intent of preventing pregnancy, the shots did pretty well. During the efficacy phase of up to 56 weeks, four pregnancies occurred among the partners of the 266 male participants. That’s a rate of 1.57 per 100 continuing users.

That compares with an effective rate of 82 percent for condoms and 78 percent for withdrawal.

Only vasectomy has better numbers, 99 percent, but not everyone is ready for that permanent solution.

Despite the problems, the shots earned high marks from participants.

At the conclusion of the trial period, more than 75 percent of participants reported being willing to use this method of contraception.

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Dr. Jamin Brahmbhatt, a urologic surgeon at Orlando Health in Florida, is on the front lines of the male contraception discussion.

“We need more options,” Brahmbhatt told Healthline. “This study is a step in the right direction. If nothing else, it gets men thinking about male contraception.”

The hormone shots limit the production of sperm.

“What happens is that it leads the testicles to shut down sperm production,” Brahmbhatt explained.

Describing himself as an advocate for men’s health, Brahmbhatt said, “I deal with this daily.”

He noted that drug companies have created many good options for women.

“It’s easier to inhibit that one egg than millions of sperm,” he noted.

Brahmbhatt thinks the hormone shot would be viable commercially.

“There would be a vast market internationally since men would [only] need a shot every two months,” he said.

He said a shot might work better than a pill, where the noncompliance rate would be higher.

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Dr. Lawrence Jenkins, a urologist at The Ohio State University Wexner Medical Center, said the shot is intriguing but may be a way off.

“It’s an interesting concept,” he told Healthline, “but it’s not close to prime time.”

He sees a lot of patients who would be interested in reversible contraception — and so far this is not.

He thinks a method like Vasalgel may have more promise. That’s a gel that puts a barrier in the vas deferens, the duct that conveys the sperm from the testicles to the urethra.

Jenkins said Vasalgel has been studied in Europe and the company wants to do a study in the United States.

“That gel affects delivery [of sperm], not production,” Jenkins said.

To reverse its effect, it can be washed out.

“There’s interest out there,” he noted, saying that 92 percent of couples have said that men and women should share contraceptive responsibility.

But the biology is complicated.

The eggs are already there, he said, while sperm are newly produced every three months.

“To suppress production [of sperm] is more complicated than to prevent an egg from being released,” Jenkins said.

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For now, the burden remains on the woman.

Allan Pacey, Ph.D., a professor of andrology at the University of Sheffield, England, spoke to that point when he told the BBC, “There is certainly an unmet need for an effective reversible contraceptive for men, along the lines of the hormonal contraceptive for women.”

Pacey is on the faculty of the university medical school. He noted that nothing that has been tested so far has become a commercial reality.

But he thought it significant that three-quarters of the test population would be willing to take the shots again.

“So perhaps the side effects weren’t all that bad after all,” he concluded.

A similarly optimistic view came from Dr. Mario Philip Reyes Festin, a study co-author, and World Health Organization researcher.

He told Science Daily, “The study found it is possible to have a hormonal contraception for men that reduces the risk of unplanned pregnancies in the partners of men who use it.”

“Our findings confirmed the efficacy of this contraceptive method previously seen in small studies,” he added. “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.”