Kissing is, for the most part, a safe activity; most STIs are not commonly passed; however, herpes (sometimes referred to “cold sores”) is one that can be easily passed.
Sexually transmitted infections (STI) like gonorrhea and human papillomavirus (HPV) can also be passed with deep kissing, but they are not commonly passed this way.
Syphilis sores can be in the mouth, so it is possible to get syphilis from deep kissing. However, it is not a common way for syphilis to be passed.
Have a look at our ‘Know your chances’ and ‘Types of STIs’ pages to learn more about these STIs and how they are passed.