Couples in Manchester and Edinburgh are among up to 450 taking part in the two-year international study.
A British man is among the first in the world to trial a male contraceptive gel as part of a two-year international study.
PHD student James Owers, from Edinburgh, told Sky News that his partner saw an advert for the project and asked him if he wanted to get involved.
He said: “I thought it was a good opportunity to really make a difference into the discourse about responsibility in terms of contraception.
“At the moment, men only really have the vasectomy or the condom – and if you want to have kids in the future, a vasectomy isn’t such a great idea and condoms are very, very ineffective.
“The recorded failure rate of condoms is 17%, so I was quite keen from a selfish perspective, to get more options and to help develop those.”
He is among 450 couples who will be involved in trialling the gel formulation, called NES/T, which includes the progestin compound segesterone acetate, in combination with testosterone.
It is applied to the back and shoulders and absorbed through the skin.
The progestin blocks natural testosterone production in the testes, reducing sperm production to low or non-existent levels.
The replacement testosterone maintains normal sex drive and other functions that are dependent on adequate blood levels of the hormone.
Mr Owers suggested remembering to apply the gel every day is not an issue and that the process is “absolutely fine”.
He described it coming in a “toothpaste dispenser” like tube – adding: “You just dispense some on your hands and you rub it into your shoulders and shoulder blades, it takes about 30 seconds, dries really quickly, doesn’t smell or anything and it’s really very little effort indeed.”
Researchers also claim that so far, the trials have proved very effective.
Mr Owers said: “It’s actually quite interesting, it takes six to 12 weeks to get your sperm count all the way down and it takes about six to 12 weeks for it to come back up again.
“So it’s quite different to the pill in as much as if you miss the pill on one day or in fact, you miss it by 12 hours, there is some non-zero chance that you will ovulate.
“But if I was to miss taking this for an entire week, I would still be clinically infertile, so the risk here is quite different from the pill.”
The study is being led in the UK by Saint Mary’s Hospital, part of Manchester University NHS Foundation Trust, and the University of Edinburgh.
The project is being funded by the US National Institute of Health and led by the Los Angeles Biomedical Research Institute and the University of Washington School of Medicine.
Dr Cheryl Fitzgerald is among those behind the study and told Sky News that so far it was going “very well”.
She said: “We are going to be looking at these men very closely throughout the duration of the trial to check that their sperm counts stays low.
“Obviously, if that doesn’t happen, we will tell couples they need to use other forms of contraceptive, but certainly the evidence we’ve got so far, is that this is very very effective and it really does, really really suppress sperm counts, so I think it will be highly effective.”
Dr Fitzgerald added: “I think women have got quite a few choices and I think men need to have a choice.
“Also, there are lots of women who can’t take different forms of contraception, so contraception becomes quite an issue, so I think anything that can be added into that must be good.”
:: Researchers are still looking for men aged between 18 and 50, who are in a stable relationship with a woman aged between 18 and 34, to take part, by calling 0131 242 2669 (Edinburgh) or 0161 276 3296 (Manchester)