According to an enthusiastic article on the Internet, “The Best Birth Control In the World Is For Men.”
It’s called RISUG: Reversible Inhibition of Sperm Under Guidance. It involves a minor surgical procedure in which the vas deferens is exposed and pulled outside the scrotum by the same techniques used for a vasectomy. A copolymer, powdered styrene maleic anhydride (SMA, for which the method was previously named) combined with dimethyl sulfoxide (DMSO) is then injected into the vas deferens. The polymer coats the walls of the vas and kills the sperm as they swim by. The mechanism is not understood, but the developer thinks the polymer’s mosaic of positive and negative charges causes the membranes of the sperm to burst, rendering them immotile.
RISUG is rapidly effective: in a phase II clinical trial in India, viable sperm were absent as soon as 5 days after the procedure. They say there have been no pregnancies in the first months “other than a handful of cases in which the RISUG was not injected properly.” (One wonders how they determined that it was not injected properly: by the fact that pregnancy occurred? Could this be just a rationale to explain away failures? Or to spare patients the embarrassment of discovering the wife had another sperm donor?) The contraceptive effect is said to last for a decade or more; it might require repeat injections every 10 years.