From Scientific American:
A joke among researchers in the field of male contraception is that a clinically approved alternative to condoms or vasectomy has been five to 10 years away for the past 40 years. The so-close-yet-so-far state of male contraceptive development has persisted in large part because of three serious hurdles: the technical challenges of keeping millions of sperm at bay, the stringent safety standards that a drug intended for long-term use in healthy people must meet, and, ultimately, the question of whether men will use it. Any sex-ed grad can tell you: the only two effective contraceptives for men today are condoms and vasectomy. Condoms have been around for at least 300 years, with early versions made of animal intestines. Today's rubber prophylactics are relatively cheap and widely available, offer bonus protection against sexually transmitted infections, and are 98 percent effective against pregnancy if used properly. On the other hand, surgery to cut the vas deferens (sperm ducts) is nearly foolproof in pregnancy prevention but is usually considered irreversible and tantamount to sterilization. “It's appalling that besides condoms men only have a surgical nonreversible method,” says Regine Sitruk-Ware, a reproductive endocrinologist at the Population Council in New York City.
For decades the promise seemed to lie in a hormonal approach—an analogue to the female birth control pill—that would adjust the hormones controlling sperm production. Inconsistent results among men and side effects associated with long-term testosterone use have, however, led some researchers lately to set their sights elsewhere. Newer, nonhormonal methods target various developmental nodes in the formation of sperm, their motility and their egg-penetrating capabilities. There is also work on a form of reversible vasectomy which involves blocking the vas deferens with a polymer gel that may later be dissolved.