Scientists trialling an experimental injected form of male contraception have shown that it's almost completely effective in preventing pregnancies – getting us closer to finally having a male equivalent to female contraceptives like the pill and IUDs.

But while the hormone shot was effective for almost 96 percent of men testing it, side effects reported by some participants saw the trial ending earlier than planned, and could mean that the contraception never actually sees the light of day.

"If you're comparing it to other reversible male methods, it's far better than the condom and it puts it in the same ballpark as the pill," clinical reproductive scientist Richard Anderson from the University of Edinburgh in the UK, told Hannah Devlin at The Guardian. "The results provide us with confidence that this can be done."

Anderson and fellow researchers recruited 320 healthy male participants aged between 18 and 45 to trial the injection, designed to lower sperm counts during the course of treatment.

The shot consists of two hormones: progestogen, which affects sperm production by acting on the pituitary gland, and testosterone, to mitigate the testosterone-reducing effects of the progestogen.

The men testing the contraceptive were all in long-term monogamous relationships with female partners, and had healthy sperm counts at the outset of the trial.

Each man received two injections every eight weeks over a period of 56 weeks. Their partners stopped using any kind of birth control once the injection took effect, by lowering the man's sperm count to less than 1 million/ml.

Semen samples showed that 274 of the participants reached this level within 24 weeks, and the contraceptive was found to be effective in almost 96 percent of continuing users.

But four pregnancies resulted during the experiment, showing the injection is still not as reliable as the female pill – which is up to 99.9 percent effective.

In the absence of any other male contraceptives tested in clinical trials, though, the researchers are claiming the efficacy achieved so far as a victory.

"The study found it is possible to have a hormonal contraceptive for men that reduces the risk of unplanned pregnancies in the partners of men who use it," said researcher Mario Philip Reyes Festin from the World Health Organisation in Geneva, Switzerland in a press release.

Of more concern are the side effects that some men reported during the trial, which saw 20 participants ultimately drop out. These symptoms included depression, mood disorders, injection site pain, muscle pain, increased libido, and acne.

A suicide commited by one of the participants during the trial was found to not be related to the use of the treatment. But the drug could have been linked to one intentional paracetamol (acetaminophen) overdose, and one case of depression, in addition to other adverse effects.   

In light of these side effects – and despite the fact that three quarters of the men indicated they would be happy to keep taking the drug – some scientists who weren't involved with the trial have shown reservations over the jab.

"[T]he fact that so many side effects were observed in the men who were taking part in the trial is of concern," andrologist Allan Pacey from the University of Sheffield in the UK said in a statement.

"For a male contraceptive to be accepted by men (or women) then it has to be well tolerated and not cause further problems. For me, this is the major concern of this study."

While there are currently no male contraceptives on the market – with the exception of condoms – there's no shortage of research in the area.

One of the most publicised efforts is a drug called Vasalgel, which is currently undergoing clinical trials and – provided testing pans out – could be available to the public as early as 2018.

And just this week, researchers in the UK announced success with a new peptide-based treatment, which they say can affect sperm's ability to swim – with potential for use in fertility boosters in addition to contraceptives.

"The results are startling – and almost instant," researcher John Howl from Wolverhampton University told Stephen Adams at the Daily Mail. "When you take healthy sperm and add our compound, within a few minutes the sperm basically cannot move."

In terms of the injection study, the researchers acknowledge there's more work to do, but they're confident we're getting close.

"More research is needed to advance this concept to the point that it can be made widely available to men as a method of contraception," said Festin.

"Although the injections were effective in reducing the rate of pregnancy, the combination of hormones needs to be studied more to consider a good balance between efficacy and safety."

The findings are reported in The Journal of Clinical Endocrinology & Metabolism.