Assessment of Metformin for Restoration of Immune Homeostasis in HIV+ and HIV- Individuals With a History of Injection Drug Use

This study is testing whether metformin — a widely used diabetes medication — can help restore healthy immune system function in people who have a history of injection drug use, with or without HIV. The study also includes two approved vaccines (for mpox and pneumococcal disease) to measure immune response. **You may be eligible if...** - You are between 18 and 64 years old and weigh at least 110 lbs - You have a history of injecting opioids, amphetamines, or cocaine within the past 10 years - Your BMI is between 18.5 and 40 (or slightly above with medical approval) - You are willing to take metformin orally and receive the two study vaccines - You are HIV-positive or HIV-negative (both groups are included) **You may NOT be eligible if...** - You currently have uncontrolled diabetes or are already on metformin - You have significant kidney problems (metformin requires normal kidney function) - You have a condition that makes vaccination unsafe - You are pregnant or planning to become pregnant during the study Talk to your doctor to see if this trial is right for you.