What effect will most likely result from a combination of drug abuse and unprotected sex with different partners?

Increased risk of developing a sexually transmitted infection.

Explanation:

Combining drug abuse and unprotected sex with different partners can increase the risk of developing a sexually transmitted infection. This is because the practice of unprotected sex, ie without the use of condoms, is the most common way to contract a sexually transmitted infection. In addition, drug abuse is often done with injecting drugs, where users use the same needle and can pass infections on to each other.