Researchers test an experimental male birth control pill designed to immobilize sperm

Atlanta - The first birth control pill for women debuted in 1950.

But, 73 years later, researchers are still trying to find a form of oral contraception that will work for men.

In New York, Cornell University postdoctoral fellow Melanie Balbach is part of a team working on what she hopes is the next big thing in male contraception: a short-acting pill men could take on demand, or only when men want to have sex.

"For us, it makes way more sense," Balbach says. "Why take a contraceptive when you don't need it?"

Women typically take daily birth control pills.

Balbach is trying to develop a pill men could 30 minutes before initiating sex, that would keep working for up 12 to 24 hours.

"So, the idea is that the men really only take out birth control whenever they need it, whenever they want to have intercourse," she says. "Then, whenever they don't want to have sex, they wouldn't be on birth control."

The pill, which has been tested in mice and will now be tested in rabbits, is a sperm inhibitor that uses an enzyme to temporarily immobilize sperm, blocking its ability to swim.

"So then, the sperm are not swimming anymore," Balbach says. "So, it's kind of the light switch on, the sperm is blocked. They're not switched on, and they're not starting to swim."

When sperm cannot swim, she says, they cannot get up through the cervix to fertilize an egg.

"They're just getting trapped in the vagina, and they're not swimming up the female genital tract," she says.

Other birth control pills in development tend to focus on lowering sperm count, Balbach says, but this is the first to block sperm from being able to swim up through the cervix.

"The vagina is a rather hostile environment," she says. "So, what happens is the sperm don't get out of there quick enough. The immune system comes in and degrades the sperm, and then, also, the pH in the vagina gets too acidic, which the sperm don't like, and that also kills them all."

Balbach says the drug worked well when it was injected into the mice, blocking pregnancy for several hours.

She now plans to study the drug in rabbits.

If the animal studies show the pill is safe and effective, she says, they could begin clinical trials with men in a couple of years.

But, Balbach hopes an on-demand oral contraceptive for guys will be worth the wait.