The gel that consists of the antiretroviral drug raltegravir could potentially help to protect against HIV even if it's applied after sex (as long as it's within three hours).  While the formula hasn't been tested on humans yet, researchers hope that it "could be used for H.I.V. prevention like Plan B or the morning-after pill for contraception." The effectiveness isn't quite 100 percent, as tests conducted on macaques found that one of the six given the gel still got infected with H.I.V.

These "microbicide" treatments have shown promise in previous trials, but had to be used consistently for too long in order to be really effective. But perfecting the formula could benefit women in countries that "have little protection against domestic violence or rape, because they could apply it surreptitiously after a partner fell asleep or a clinic could administer it after a rape."

[NY Times]