Factor #2: Substance Use
I work with many patients whose substance abuse has particularly devastating consequences on their medication adherence. This is especially true for methamphetamine. Most meth users take the drug over several days, during which genuine commitments (like using condoms or taking one's HIV meds) are abandoned. Some people make heroic, but usually unsuccessful, efforts to take their HIV meds (setting mobile phone alarms, asking friends to prompt them). Others just give up while partying.
Recreational drugs can also interact with HIV meds in a variety of ways, which may cause a dangerous increase in blood levels (this can lead to an overdose of the recreational drug) or the rapid metabolism of the antiviral, creating an opportunity for mutations that lead to resistance.
Anyone starting HIV meds needs to seriously consider their recreational drug use and its potential impact on their ability to maintain their medication regimen. HIV meds don't mix well with recreational drugs.