Yes, as far as HIV is concerned, says a 2001 University of California study, seconded by a 2002 Spanish survey reporting zero HIV infections after 19,000 instances of unprotected oral sex between HIV positive and negative heterosexual partners. But in 2000, the Options Project found that 8 out of 102 infections between guys likely took place through oral sex.

Experts think saliva is a natural HIV inhibitor, but cuts and sores as well as semen or vaginal blood in the mouth can up risk for HIV and other sexually transmitted diseases.