Throughout the past two decades, researchers have faced some major hurdles; primary among them are side effects and funding. Late last year, researchers published the results of a phase II clinical trial, commissioned by the World Health Organization, that evaluated a two-hormone drug designed to lower sperm count in men. The drug was effective, but the trial ended early because there were too many side effects and the hormone combination didn't work effectively in everyone.
In 2012, researchers from the National Institutes of Health tested an injectable contraceptive of two hormones; their results were promising, and they will soon begin testing it in a larger population
Other compounds also disrupt the production of sperm, but without hormones. CatSper, for example, can alter the function of sperm so they won't fertilize an egg without disrupting the rest of the man's reproductive system, though this is still in the research stage.
JQ1, a compound initially used to block a protein implicated in various forms of cancer, was found to disrupt the production and motility of sperm; an article published last year in MIT Tech Review notes that it has not yet been tested in humans.
A researcher at the University of Washington is testing a version of a compound called WIN 18,446, which interferes with the vitamin absorption necessary to make sperm. It was first tried in humans in the 1950s but was found to cause harmful side effects when combined with alcohol, according to Bloomberg Businessweek.
More from Modern Medicine:
Medical emergency: ER costs skyrocket, leaving patients in shock
Venture capitalist wages war against Parkinson's after his own diagnosis
The story behind the new sickle cell drug 25 years in the making
Another compound, called H2-gamendazole, prevents sperm from reaching their mature (and thus virile) form. In 2015 the compound was still being tested in animals; the U.S. Food and Drug Administration had previously determined that more research was needed to make sure the compound didn't have any additional effects on women's bodies, Wired reports.
But to some experts these sorts of chemical interventions are less than ideal. They take a long time to kick in and have more side effects than those that physically block the passage of sperm or interfere with how it fertilizes an egg, says Aaron Hamlin, the executive director of the Male Contraception Initiative, a nonprofit organization working to bring new male contraceptives to market. "Our organization is not a fan of approaches that stop sperm production. We would rather not take that route because of the inherent delays," he says.