File photo: A previous jab, using hormones to disrupt sperm production in the brain, was found to cause side effects including depression and muscle pain – leading 20 men to drop out of a year-long trial. Picture: Reuters

London - A male contraceptive jab has been found to prevent pregnancy for up to two years.

The one-time injection, which has been described as a "reversible vasectomy", is more effective than the female Pill, and comes as a new breakthrough in the search for a male equivalent.

A previous jab, using hormones to disrupt sperm production in the brain, was found to cause side effects including depression and muscle pain – leading 20 men to drop out of a year-long trial.

But the new non-hormonal injection, which had a 100 percent success rate in a study on monkeys, did not appear to cause any side effects. It could provide another option for the more than 10 000 men a year in England who have vasectomies.

The Vasalgel jab works by injecting a gel into the vas deferens – the tube which transports sperm from the testicles to the urethra – to block it. As well as being less painful than "the snip", a previous study on rabbits found it to be reversible. Surgery to reverse a vasectomy costs thousands of pounds, and has a success rate as low as 30 percent.

It could be trialled on humans as early as next year, after working in male rhesus monkeys, which failed to impregnate female monkeys over a period of up to two years.

Professor Adam Balen, chairperson of the British Fertility Society, said: "This is an interesting technique that achieves a reversible 'vasectomy' by blocking the passage of sperm with a substance that later can be flushed out. If free of side effects, then this novel approach has the potential for great promise as a male contraceptive."

The study, by the California National Primate Research Centre, monitored a group of 16 monkeys given Vasalgel for at least one breeding season. Typically 80 percent of the females housed with them during such a period would become pregnant.

But they found that over an average of almost 1.2 years – and up to two years for some monkeys – there were no pregnancies, reported the journal Basic and Clinical Andrology. This is more successful than the Pill, which is around 99 percent effective.