Nonoxynol-9 is widely used in contraceptives for its spermicidal properties (it kills sperm on contact). It has been promoted as a backup method for avoiding pregnancy and to protect against STIs in the event of condom failure.
In the late 1990s, studies have found that condoms with nonoxynol-9 can irritate the lining of the rectum/ arse leading to increased risk of STI transmission, including HIV. Most condom manufacturers in the UK have stopped using N-9, but avoid condoms described as spermicidally lubricated.
Lubricants containing N-9 may enhance rectal transmission of HIV and other STIs | Contraception, 2004 | 70(2):107-110
David M Phillips, Kristin M Sudol, Clark L Taylor, Laura Guichard, Robert Elsen, Robin A Maguire
Confusion over use of nonoxynol-9 among gay men in San Francisco despite public warnings | 2003 | NAM Aidsmap
Nonoxynol 9 | Wikipedia