US scientists have developed an easy to apply gel that could protect against genital herpes for up to 10 days.
Condoms prevent a number of sexually transmitted infections
Using a condom with a spermicide also protects against the virus, which also causes cold sores.
But there is currently no protection that can last for days without reapplication or as an alternative for people who do not want to use condoms.
The early findings, published in New Scientist, could be of particular help to the developing world, experts said.
Genital herpes is caused by the herpes simplex virus. In the UK, about a fifth of people who are sexually active are infected. But in Africa, up to 50% of people have the virus.
The virus can sit latent in cells for long periods of time and the person may not know they have it and can pass it on to partners.
When the virus is reactivated, it can cause painful symptoms and distress.
These symptoms are treatable, but there is currently no cure. Scientists have been looking at developing a vaccine, but this is still some time away. At the moment, the best way to stop the virus is to prevent transmission.
The gel, developed by Dr Judy Lieberman and colleagues at Harvard Medical School, blocks herpes transmission by destroying the virus.
The active ingredient is called small interfering RNAs.
When applied to the vagina, the gel is absorbed and remains active for at least 10 days.
So far, studies in mice have shown that the gel completely blocks infection.
Dr Lieberman told New Scientist that if it works in people too, the long-lasting protection would be a great advantage.
"The problem with microbicides is that people don't remember to use them before they have sex."
The gel could also be useful in developing countries where people may refuse to use condoms, said experts.
Marion Nicholson, director of the Herpes Virus Association, said: "This is very interesting, although it is early days yet."
She said if it worked, it would have advantages over condoms and spermicides or microbicides.
For example, a woman would be able to apply it once and not have to worry about reapplying it again until 10 days later, where as condoms and microbicides have to be used each time the couple has intercourse, she said.
"How fantastic would that be?" she said.
She said, in the future, it might mean women could still protect themselves against genital herpes even when their partner refused to wear a condom.
But she stressed that it was important to use condoms to prevent a number of sexually transmitted infections, including HIV.
Genital herpes can also be caught from oral sex if the person has an active cold sore, she added.