Psilocybin Therapy for Methamphetamine Use Disorder and HIV

The goal of this clinical trial is to learn whether it is possible to use psilocybin in combination with motivational support therapy to treat moderate-to-severe methamphetamine use disorder (MeUD) in people with HIV who are seeking to stop using methamphetamine. The main questions it aims to answer are: * Do people with HIV and MeUD find psilocybin with motivational support therapy feasible and acceptable as a potential treatment? * Is psilocybin safe and tolerable among people with HIV and MeUD? Participants will: * Be randomly assigned to receive a single monitored dose of either 25 mg (higher dose) or 5 mg (lower dose) psilocybin * Have 3 preparation and 3 integration motivational support therapy visits before and after the psilocybin dosing session. * Report their methamphetamine use prior to, during, and up to 3 months following the intervention * Optionally receive one additional open-label 25 mg psilocybin session after the 4-week assessment, if eligible