UnitedHealthcare will restrict coverage of Biogen’s Aduhelm, the nation’s largest health insurance company notified providers Sunday.

The UnitedHealth Group subsidiary will only cover the costly Alzheimer’s disease treatment also known as aducanumab for patients enrolled in clinical trials and will require prior authorization. The medication is “unproven and not medically necessary for the treatment of Alzheimer’s disease due to insufficient evidence of efficacy,” the insurer wrote in bulletins sent to providers.

The rules apply to the insurer’s commercial, individual and Medicare members and take effect June 1.

The company’s policy aligns with the Centers for Medicare and Medicaid Services’s recent decision to limit Aduhelm coverage to Medicare beneficiaries participating in clinical trials. Under federal law, Medicaid must cover virtually all prescription drugs that the Food and Drug Administration approves, including Aduhelm. States may impose limits such as requiring prior authorization, CMS determined.

UnitedHealthcare cited a number of studies to support its decision, including a July report by the Institute for Clinical and Economic Review that concluded patient use of the drug with supportive care did not produce a net health benefit compared to supportive care alone.

“We will continue to monitor ongoing clinical studies, as well as guidance from CMS and other agencies, and will update our clinical and coverage policies as necessary,” UnitedHealthcare said in a statement.

UnitedHealthcare is the first publicly traded health insurer to announce an Aduhelm coverage decision. Other carriers, including at least six not-for-profit Blue Cross and Blue Shield affiliates, are not covering Aduhelm. In addition, major providers such as Mount Sinai Health System and the Cleveland Clinic won’t prescribe the medicine.