Efforts to develop a long-term, reversible contraceptive for men have thus far been unsuccessful, but a recently completed study showed sufficiently promising results that further trials will no doubt soon be underway. The trial lasted 2.5 years and involved 1045 men in China. The mechanism by which this contraceptive works is the same by which hormonal contraception for women works: if you interfere with the feedback loop that regulates the production of gametes (eggs and sperm), you can turn it off. In other words, you supply a form of the hormone that the body normally makes itself, and the body stops making it, and reproductive function stops. In this case, the men were given monthly administrations of a form of testosterone, which suppressed sperm production. Side effects were minimal, and sperm production returned to normal within 6 months of stopping the drug in all but 2 cases. But don't look for this to appear at your doctor's office any time soon. I found a nearly identical study conducted in 1996 that was also very successful.
Funny, one article describing the recent study said this:
"Previous attempts to develop an effective and convenient male contraceptive have encountered problems over reliability and side effects, such as mood swings and a lowered sex drive. "
Um, I'm sorry, but haven't women using hormonal contraception been putting up with those side effects, not to mention at least 2 dozen others, for, like, 40 years?