The investigators recently conducted a double-blind, randomized controlled trial (n=60) of limited duration (12 weeks), and found that compared with placebo, oral mirtazapine, an FDA-approved antidepressant, significantly reduced meth use in those receiving mirtazapine, as determined by reduction in meth-positive urines. Sexual risk behaviors also declined significantly in the mirtazapine arm compared to placebo. Mirtazapine decreased meth use despite low adherence: by medical event monitoring system (MEMS) caps, only 48.5% of daily doses were taken. All participants received weekly substance use counseling and monthly, brief clinician-delivered adherence counseling. The investigators propose expanding upon these results by lengthening the treatment period to 24 weeks, with adherence reminders added to the counseling, and determining if efficacy is sustained up to 12 weeks after drug discontinuation. The sample size for this 9-month study is 120.

Inclusion Criteria:

1. born male, or born female and does not identify as female;
2. reports anal sex with men in the prior three months while under the influence of meth;
3. diagnosed with meth dependence by SCID;
4. interested in stopping or reducing meth use;
5. at least one meth-positive urine during screening and run-in period;
6. no current acute illness requiring prolonged medical care;
7. no serious chronic illnesses that are likely to progress clinically during trial participation;
8. able and willing to provide informed consent and adhere to visit schedule;
9. age 18-69 years;
10. baseline CBC, total protein, albumin, glucose, alkaline phosphatase, creatinine, BUN, and electrolytes without clinically significant abnormalities as determined by clinician in conjunction with symptoms, physical exam, and medical history
11. current CD4 count ≥ 200 cells/mm3; or CD4 count of 100 - 199 cells/mm3 and HIV viral load < 200 copies/mL
12. text-capable cell phone or access to email

Exclusion Criteria:

1. Evidence of current major depression by SCID;
2. history of bipolar disorder or psychotic disorder, as determined by SCID;
3. known allergy or previous adverse reaction to mirtazapine;
4. taking an anti-depressant medication within the past 30 days, including mirtazapine or a monoamineoxidase inhibitor;
5. moderate or severe liver disease (AST, ALT, and total bilirubin >= 5 times upper limit of normal);
6. impaired renal function (estimated GFR <40 ml/min);
7. currently participating in another research study;
8. pending legal proceedings with high risk for incarceration during the time of planned study participation;
9. any condition that, in the principal investigator's judgment, interferes with safe study participation or adherence to study procedures.