If promising, study results will be used to design a phase III clinical trial to determine if mirtazapine's effects on reducing methamphetamine use lead to reductions in methamphetamine-associated sexual risk. We have chosen first to conduct a 3-year intermediate-sized trial in order to determine if mirtazapine reduces methamphetamine use and whether mirtazapine demonstrates good acceptability and tolerability among a population with methamphetamine-associated high-risk sexual behaviors. If this proves to be the case, we believe our study results will provide strong support for a much larger trial to test the hypothesis that mirtazapine-driven reductions in methamphetamine use will result in corresponding decreases in sexual risk behavior. This study is therefore designed to reflect the structure of a larger HIV-risk reduction trial and includes both substance use and sexual risk behavior measures. We will enroll sexually active, methamphetamine-dependent MSM (either HIV-negative or HIV-positive) who will be randomized 1:1 to receive mirtazapine or placebo for 90 days. Because no medications have been approved to treat methamphetamine dependence, we include extensive safety parameters, as is required by the Food and Drug Administration (FDA) when testing a medication for a new indication in a new population. Participants will be seen weekly for urine drug testing and for brief substance use counseling. All will receive HIV risk-reduction counseling. Behavior will be assessed using standardized measures via audio computer-assisted self-interview (ACASI).