Refusing to provide insulin to a diabetic in prison would be unconstitutional. The Supreme Court held in 1976 that the Eighth Amendment prohibition on cruel and unusual punishment includes the right to necessary medical care behind bars. In a first-of-its-kind case last month, a federal trial judge in Boston ordered a county jail to allow an inmate to take his prescribed methadone for opioid-use disorder.

The standard of care for opioid addiction is medication-assisted treatment, which combines counseling and other supportive...