Despite the perks of deep French kissing, this practice does carry some risks. Just like regular French kissing, if one of the kissers has a cold sore, herpes can be spread. Deep French kissing can also spread additional sexually transmitted infections, including gonorrhea, syphilis, and hepatitis B.
Generally, there is no risk of spreading HIV during deep French kissing. This virus is transferred through blood rather than saliva. If the kissers have bleeding gums or open sores, however, there may be some risk.