3 STIs That Barrier Methods Can’t Reliably Prevent Bowonpat Sakaew via iStock When it comes to condoms, we cannot sing their praises enough. Condoms—both external and internal (“female”), along with dental dams—are a cornerstone in the prevention of sexually transmitted infection (STI) spread. When used correctly, condoms are highly effective in preventing most STIs, including chlamydia, gonorrhea, asymptomatic syphilis, and HIV. As a sex educator, I recommend condom use for anyone who is sexually active, including for oral, anal, and vaginal intercourse, in order to protect yourself and your partner. With all that being said, condoms are not perfect. There are some STIs that can still be spread even if you use condoms every single time you engage in sex. While most STIs (such as HIV) are spread through blood and genital fluids, there are a few that are transmissible through skin-to-skin contact. A condom on a penis only covers the shaft, which means that the scrotum, perineum, and anus are exposed. Similarly, an internal condom sits inside the vaginal canal, leaving the vulva exposed.