The promise of a reliable and safe male oral contraceptive pill just got a lot closer. Reporting in the Journal of Clinical Endocrinology and Metabolism, scientists have declared the success of a phase 1 study looking at the safety and tolerability of a new birth control pill for men.

After taking the pill for a month, the men experienced a change in hormones that would cause a drop in sperm production. Crucially, they reported relatively few side-effects – even most of the men's libidos remained unchanged – and the contraceptive effects were reversible after stopping treatment.

The researchers still need to carry out larger and longer studies, as well as tests on sexually active couples, but they hope their work could lead to a new male contraceptive option in a decade.

“Our results suggest that this pill, which combines two hormonal activities in one, will decrease sperm production while preserving libido," Christina Wang, Los Angeles Biomedical Research Institute and Harbor UCLA Medical Center, said in a statement.

"Safe, reversible hormonal male contraception should be available in about 10 years," Wang predicted.

The pill, known as 11-beta-MNTDC, contains a modified form of testosterone that has the combined actions of the androgen and progesterone. Progesterone works to block the production of hormones called LH and FSH that are needed for the testes to produce testosterone and sperm, while androgen – a male hormone – helps to counteract the drops in testosterone. The team describes 11-beta-MNTDC as a “sister compound” to another experimental male oral contraceptive, DMAU, which the same team published results about last year.

For 28 days, 30 healthy men took the pill in one of two doses of 11-beta-MNTDC; 14 men received 200 milligrams and 16 took the 400-milligram dose. Meanwhile, a further 10 healthy men took a placebo capsule for the same time.

The researchers did not actually look for changes in sperm production, as the drug would take months to have this effect. But they did note changes in hormones that suggest the men would have experienced a drop in sperm production consistent with effective contraception.

No severe side-effects were reported either, although a few of the participants reported mild fatigue, acne, or headaches. Five men also reported mildly decreased sex drive and two men described mild erectile dysfunction, however, this did not appear to affect sexual activity, according to the study. If this sounds a little worrying, it’s worth remembering the vast list of side effects that come with the already commercially available female contraceptive pill, from mood swings, headaches, and nausea to decreased libido, weight gain, and increased risk of depression.

"The goal is to find the compound that has the fewest side effects and is the most effective," added Professor Stephanie Page of the University of Washington School of Medicine. "We are developing two oral drugs in parallel in an attempt to move the [contraceptive medicine] field forward."