Lifting Medicaid Prescription Caps Boosted HIV Medication Adherence, Study Finds

When South Carolina removed its Medicaid prescription cap in the summer of 2017, the policy change quietly rewrote the daily medication routines of more than a thousand people living with HIV. A new difference-in-differences study published in the Journal of General Internal Medicine finds that scrapping the cap was associated with a measurable improvement in antiretroviral therapy adherence, with the largest gains concentrated among beneficiaries who had previously been constrained by the limit. The research, led by Fangli Geng of Brown University School of Public Health together with colleagues at Brown and the University of Southern California, offers some of the clearest real-world evidence yet on what happens when states loosen one of the most common cost-control levers in the Medicaid pharmacy benefit.

Prescription caps, sometimes called drug limits, restrict the number of prescriptions a Medicaid beneficiary can fill in a given month. States adopted them widely beginning in the early 2000s as a blunt instrument for controlling pharmacy spending, and they remain in force in a number of states today. The logic behind them is straightforward: limit the volume of reimbursed prescriptions and the state saves money. But decades of research, dating back to landmark studies in the late 1980s on Medicaid payment restrictions, have shown that these caps can disrupt therapy for chronic conditions, sometimes shifting costs elsewhere in the health system through increased hospitalizations and emergency care. For people with HIV, the stakes are particularly high, because consistent antiretroviral therapy is what keeps the virus suppressed, protects long-term health, and prevents onward transmission.

What made the South Carolina case scientifically valuable was the timing and the specificity of the policy change. On July 1, 2017, the state removed its Medicaid prescription cap outright, creating a natural experiment. The research team compared Medicaid beneficiaries with diagnosed HIV in South Carolina against similar beneficiaries in nine control states that maintained their caps throughout the 2016 to 2019 study window. To make the comparison as clean as possible, the investigators used propensity score matching to pair individuals in the policy-change state with statistically comparable people in the control states, then applied a difference-in-differences framework that nets out baseline differences between the groups and shared temporal trends. An event-study design allowed them to trace how the effect evolved month by month after the cap came off.

The study population was carefully defined. The sample included adults aged 21 to 64 years with diagnosed HIV and continuous Medicaid claims spanning both the pre-policy and post-policy periods. Beneficiaries who moved between states, lived in long-term care facilities, or were pregnant were excluded, since each of these circumstances could confound the relationship between policy and medication use. After matching, the analytic sample comprised 1,135 individuals in both South Carolina and the control states, drawn from the Transformed Medicaid Statistical Information System analytic files, the federal data infrastructure that underpins much of contemporary Medicaid research.

The headline result concerns adherence, measured as the proportion of days covered, or PDC, a standard metric that captures the fraction of a period during which a patient has medication on hand. In the post-policy period, the change in PDC was 4.6 percentage points greater in South Carolina than in the control states, with a 95 percent confidence interval of 2.7 to 6.6 percentage points. In practical terms, beneficiaries whose therapy had been constrained by the cap filled their antiretroviral prescriptions more reliably once the limit disappeared, closing a meaningful fraction of the adherence gap that caps are believed to create. Adherence of this kind is not an abstract quality measure; prior work has linked higher PDC to achieving viral suppression, the clinical goal that underpins both individual health and the public health benefit of HIV treatment.

The event-study analysis added temporal texture to the finding. The adherence improvement did not appear uniformly across the follow-up period. Instead, it peaked at 8.4 percentage points roughly nine months after the cap was removed, suggesting that beneficiaries needed time to re-establish their refill patterns, work through accumulated prescription needs, and coordinate care with providers before the full effect materialized. Notably, the effect was driven by individuals whose baseline prescription fills had been below the cap, the group for whom the policy constraint was most plausibly binding. That pattern is exactly what one would expect if the cap itself, rather than some unrelated change in South Carolina, was responsible for suppressed medication use.

Alongside adherence, the study tracked prescription fills and days’ supply, both overall and for HIV-related medications specifically. Cap removal was accompanied by increases in HIV-related prescription fills and days’ supply, consistent with beneficiaries obtaining more of the antiretroviral regimens and supporting medications they needed. These utilization shifts represent the direct mechanical consequence of lifting a quantity restriction: when the ceiling rises, fills that would have been deferred, split, or foregone become possible. For a population that often manages multiple medications, including treatments for comorbid conditions alongside antiretroviral therapy, the removal of a global prescription limit can unblock access across an entire regimen, not just a single drug.

Perhaps the most sobering finding is what did not change. The researchers examined inpatient admissions, outpatient evaluation and management visits, and emergency department visits, and found no consistent changes in health care utilization following cap removal. This null result matters for the policy debate. A common argument for lifting drug caps is that improved medication access will pay for itself by reducing downstream acute care. In this study, at least over the observed window, that offset did not materialize in measurable utilization shifts. The authors are careful in their interpretation: removing the cap improved HIV-related prescription fills, days’ supply, and ART adherence, but achieving optimal adherence and improvements in health outcomes and health care utilization may require additional strategies beyond the policy change alone.

That interpretation aligns with a broader clinical reality. Adherence to antiretroviral therapy is shaped by far more than prescription availability. Housing instability, substance use, mental health conditions, transportation barriers, stigma, and the complexity of modern polypharmacy all influence whether people take their medications as prescribed. Real-world studies of adherence among people with HIV in the United States have repeatedly documented that a substantial share of patients fall short of the levels associated with reliable viral suppression, even in systems without fill limits. A cap removal lowers one structural barrier, and the 4.6 percentage point adherence gain shows that the barrier was real, but it does not address the others. The study’s own framing suggests that policy makers who want viral suppression and reduced acute care use should pair pharmacy benefit reform with adherence support services, case management, and interventions targeting the social determinants that drive missed refills.

The findings also carry weight for the many states that still maintain prescription caps. Prior research has associated state drug caps with fewer Medicaid-covered prescriptions for opioid use disorder treatment, reduced access to medications among young adults with disabilities, and disrupted therapy for schizophrenia and depression, building a consistent cross-disease picture of caps as policies with clinical costs that are easy to overlook in budget negotiations. Because Medicaid is the largest source of coverage for people with HIV in the United States, the population studied here sits at the intersection of two policy domains, disability and infectious disease, where fill limits can have outsized consequences. The South Carolina experience, analyzed with a rigorous quasi-experimental design and funded by the National Institutes of Health, provides a template for evaluating similar reforms elsewhere. As states continue to weigh pharmacy cost containment against access, the evidence from this natural experiment suggests that lifting caps can meaningfully improve medication-taking for people living with HIV, even if the full promise of that improvement, in viral suppression and avoided hospital care, depends on supports that go beyond the pharmacy counter.

Subject of Research: Effects of Medicaid prescription cap removal on antiretroviral adherence and health care use among people with HIV

Article Title: Medication Adherence and Health Care Use After Medicaid Prescription Cap Removal Among People with HIV: a Difference-in-Differences Study

Article References: Geng, F., Lu, Y., Santostefano, C. M., Lu, X., Lee, Y., Dow, P. M., Shireman, T. I., & Galarraga, O. (2026). Medication Adherence and Health Care Use After Medicaid Prescription Cap Removal Among People with HIV: a Difference-in-Differences Study. Journal of General Internal Medicine. https://doi.org/10.1007/s11606-026-10794-9

Image Credits: AI Generated

DOI: 10.1007/s11606-026-10794-9

Keywords: Medicaid, prescription caps, HIV, antiretroviral therapy, medication adherence, difference-in-differences, health policy, South Carolina, health care utilization, proportion of days covered, pharmacy benefit, viral suppression