Drug use is a varied practice and encompassing experimental, recreational and problematic use and its relationship with sexual health and HIV needs to be recognised as broadly diverse
Experimental or occasional drug use as with alcohol may reduce people's inhibitions and may result in peoples adopting less safer sexual practice, though there is some evidence to suggest that if people use condoms as protection for sexual activity this does not alter no matter what the level of in-toxification. There may be occasions when this choice is reduced, notably with drug drink spiking and unsolicited sexual aggression.
As with experimental - but noting with regular e.g. weekend drug takers there may be an impact for those already HIV infected noting some combination therapies react with recreational drugs and also weekends of serious clubbing e.g. can impact on peoples overall well being.
The most obvious link as with Hep B and Hep C is sharing of injecting equipment. Though it can be noted that with robust NX (Needle Exchange) schemes since 1980s the HIV drug users numbers in the UK are much reduced than earlier expectations and comparable areas in the US or Europe. Positively this proves problem drug users despite aspects of chaotic nature can alter and retain health maximising behaviour. Drug users who are HIV positive may also because of the chaotic nature of their lives may find it difficult to keep up a combination treatment regime.