A recent clinical trial published in the Journal of Endocrinology and Metabolism produced promising results for a male contraceptive injection, but was halted early due to reports of side effects.

The study, which followed 320 men and their partners, found that administering testosterone and progestogen hormones effectively prevented pregnancy in 96 per cent of users. The few pregnancies reported occurred within the first weeks of the treatment—suggesting that greater success could be achieved with an extended “suppression” phase of treatment, in which the injected hormones reduce sperm count.

The trial stopped enrolling new participants sooner than planned after many reported a number of adverse side effects, including mood disorders and acne. Professor Bernard Robaire of the McGill Departments of Pharmacology & Therapeutics and Obstetrics & Gynecology, who has carried out extensive research into male fertility, however, warns against jumping to conclusions about a trial with no placebo.

“You have to have a control group,” Robaire said. “Those men, I can guarantee you, would report a whole series of side effects. Every study that has had placebo groups has had placebo effects, anywhere between 30 and 70 per cent. So, in this way, this study design is conceptually flawed.”

He highlighted that the side effects described are difficult to quantify objectively.

“The higher libido, for example, that is a really subjective point,” Robaire said. “How do you assess whether the treatment actually caused it? Unless you did this in a double-blind manner, where neither the patient nor the investigator knew whether they were getting the drug, then to me these data have very little meaning.”

Despite the reported side effects, at the end of the study, 75 per cent of participants reported being willing to use the contraceptive, which bodes well for the future of male birth control. Although the biological principle of the study has proved effective, Robaire points out that logistical improvements must be made before the contraceptive becomes available.

“I think the concept of testosterone enhanced with something else will work as a male contraceptive,” Robaire said. “What you need is a better method of administration. What we propose, and have used, actually, are subdermal implants. In fact, they were the basis for the development of Norplant as a female contraceptive.”

A subdermal implant, like Norplant, would side-step the inconvenience of the biweekly injection used in the study.

“It’s been shown that you can put a crystalline steroid in plastic tube and it melts out of the membrane, goes into the circulation, and can last for a long time,” Robaire said. “That works very well rabbits, mice, and monkeys—the size of the tube used is about three millimeters in diameter and four centimeters in length. [For males, the subdermal implant] would have to be the size of a pencil.”

It is clear, therefore, that the realistic viability for a contraceptive implant for men depends on much needed biosynthetic advances. Development of a more bioactive analog of testosterone, for example, could make a smaller implant viable—as could improving its release rate.

Research into non-hormonal methods of birth control is also promising. Sperm cells rely on calcium for fertilization. Reduced fertility has been seen in men taking calcium channel blocking medication—sparking research into its use as a contraceptive, with encouraging results.

Another current line of research investigates the role of retinoic acid (vitamin A) in sperm cell production. Multiple studies suggest that manipulation of the breakdown and synthesis of the compound in the testis could be engineered to prevent conception, an approach currently in the animal trials stage.

So, will we see a male birth control pill in pharmacies in the next few years? Most likely not. But, for the generation conceived in its absence, it looks promising.