An experimental new type of male contraceptive that blocks sperm flow with a gel has been successful in monkey trials.
Vasalgel acts as a physical barrier once injected into the tubes that sperm would swim down to the penis.
The company behind it says a two-year trial, published in Basic and Clinical Andrology, shows the gel works and is safe - at least in primates.
It hopes to have enough evidence to begin tests in men within a few years.
If those get funding and go well - two big "ifs" - it will seek regulatory approval to make the gel more widely available to men.
It would be the first new type of male contraceptive to hit the market in many decades.
Male birth control
At the moment, men have two main options of contraceptive - wear a condom to catch the sperm, or have a sterilising operation (vasectomy) to cut or seal the two tubes that carry sperm to the penis from where they are made in the testicles.
Vasalgel has the same end effect as vasectomy, but researchers hope it should be easier to reverse if a man later decides he wants to have children.
In theory, another injection should dissolve the gel plug.
That worked in early tests in rabbits, but the researchers have yet to prove the same in monkeys and man.
The idea behind Vasalgel is not new.
Another experimental male birth control gel - RISUG (reversible inhibition of sperm under guidance) - that works in a similar way to Vasalgel is being tested in men in India.
Unlike RISUG, Vasalgel is not designed to impair the swimming sperm.
It merely blocks their path while still letting other fluid through, according to the manufacturer.
Both gels are given as an injection, under anaesthetic, and are meant to offer long-acting contraception.
The monkey trial
The University of California researchers tested the gel on 16 adult male monkeys, 10 of whom were already fathers.
The monkeys were monitored for a week after getting the injection and were then released back into their an enclosure to rejoin some fertile females.
Mating did occur, but none of the female monkeys became pregnant over the course of the study, which included two full breeding periods for some of the animals.
Few of the male monkeys had side-effects, although one did need an operation because the injection did not go to plan and damaged one of his tubes.
Allan Pacey, professor of andrology at the University of Sheffield, said: "The study shows that, in adult male monkeys at least, the gel is an effective form of contraception.
"But in order for it to have a chance of replacing the traditional surgical method of vasectomy, the authors need to show that the procedure is reversible."
He said there had been very little commercial interest from pharmaceutical companies in this kind of a approach.
The non-profit company researching Vasalgel, the Parsemus Foundation, has used grants and fundraising to get this far.
Prof Pacey said: "The idea of a social venture company to develop the idea is intriguing.
"I would imagine there is a worldwide market for a new male contraceptive, but trials in humans and more long-term safety data are required before we will know if it is a success."
This type of contraceptive wouldn't protect against sexually transmitted infections such as HIV.
But in terms of willingness, experts believe men would be up for trying new contraceptives, such as a gel.
Dr Anatole Menon-Johanssonm from the sexual health charity Brook, said: "Some men do want to be part of the solution and do their part.
"If you can have more options available then maybe more men would go for it."
He said the idea of a "reversible vasectomy" was desirable, whereas asking some men to take hormones to control their fertility might be "a big ask".
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