Spermicide is a substance that aids in contraception, but does not affect the transmission of STDs, by eliminating sperm in the vagina. Common spermicides’ include nonoxynol-9, an organic compound that immobilizes sperm, and benzalkonium choloride, a biocide. Both compounds can cause allergic reactions in some users, but both are also considered generally safe for use in condom, personal lubricant gels and other contraceptive products.