Dr. Aaron Richterman, who has treated HIV patients in Philadelphia for years, said the study grew out of a question he heard repeatedly in clinic: how patients were supposed to take their medicine when they did not have enough to eat. He gave patients special pill bottles that electronically track whether they take their doses, then compared adherence across the monthly Supplemental Nutrition Assistance Program cycle.

The pattern was clear. In the weeks after SNAP payments land, patients followed their regimens more closely. As funds dwindled toward the end of the cycle, the habit fell off.

The timing of SNAP disbursements, which are set by the date a person applied, effectively created a natural randomized trial, Richterman said, allowing researchers to isolate the effect of having money for food on medication behavior.

Daily HIV medication, when taken consistently, suppresses the virus and allows patients to live essentially normal lives. Missing enough doses can cause the amount of virus in the blood to rise, raising the risk of complications and of transmitting the infection to others.

Richterman described the drugs as a "biomedical miracle," but said social problems such as hunger and housing instability can undermine treatment even when patients have the medicine they need. Ensuring patients are connected to the services they qualify for, he said, can matter more than any counseling a doctor provides.

Dr. Sam Albanesi, an infectious disease physician at the Jonathan Lax Treatment Center at Philadelphia FIGHT, a health services and advocacy organization in Center City, said the findings match what he sees in his own clinic. Albanesi, who was not involved in the research, said he often learns a patient is struggling with food or housing only after tests show the virus has rebounded.

"It highlights that so many things outside of health care and medicine significantly impact our patients' health, probably beyond what my medications actually do," Albanesi said. "I keep people living longer and healthier lives, but without access to things like food, water, shelter … my patients really, really struggle."

He called this a "really exciting era in HIV medicine," with daily pills and long-acting injectable treatments available, but said basic needs still determine whether patients can benefit from them.

The research comes as the safety net around those patients is shifting. Last year the federal government began requiring SNAP participants to document that they are working, in school or volunteering, a change that has already pushed thousands of Philadelphians out of the program because they could not produce the required paperwork.

Albanesi said he is watching an even bigger change set to take effect in January, when Medicaid will require some recipients to renew coverage more frequently and meet work requirements. About half of his patients are enrolled in Medicaid, he said, and he expects the added hurdles to cause some to fall out of care.

Richterman's team is now extending the SNAP research to other chronic conditions, including diabetes and high blood pressure, to see whether the same monthly cycle affects management of those diseases.