If you're not in a mutually monogamous relationship, doctors still strongly urge patients to wear condoms in addition to taking PrEP, since they'll protect you against STIs like syphilis, chlamydia, gonorrhea, and hepatitis C, which Carmody says gay and bisexual men are particularly at risk for.
If you're in a mutually monogamous relationship with someone who is HIV-positive, the decision to use condoms is a bit more complicated. If your partner is taking antiretroviral therapy (ART), then they're likely taking several HIV medicines, which work together to prevent the virus from multiplying (the new particles are called copies) and reduce the amount of HIV in the body. If your partner's viral load is undetectable, then ART has lowered the amount of HIV to less than 40 to 75 copies per 1 mL of blood, which means there's a very low risk of transmitting HIV. Because of this added protection, there's also technically a very low risk of transmitting HIV when one partner is on ART and the other is on PrEP, says Carmody.
Still, that doesn't mean you shouldn't use them. No medication can completely eliminate the risk of HIV infection, she says, and condoms provide an added layer of protection, especially since HIV can be present in higher levels in genital fluids when compared to the blood — even more so if you have an STI — and viral load can go up between viral load tests.