The Supreme Court on Monday sided with two doctors challenging their convictions on drug distribution charges for over-prescribing opioid medications in a decision that could make it harder for federal prosecutors to prove such cases against licensed physicians.

The court was unanimous in ruling for the two doctors but split 6-3 on narrower legal issues in the decision. The justices stopped short of overturning the convictions, instead sending them back to the lower courts to reexamine the legal challenge in light of Monday’s decision.

Justice Stephen Breyer wrote the decision for the majority, ruling that in cases where someone who is authorized to prescribe medication is being prosecuted under the Controlled Substances Act, prosecutors must prove “beyond a reasonable doubt that the defendant knowingly or intentionally acted in an unauthorized manner.”

“We normally would not view such dispensations as inherently illegitimate; we expect, and indeed usually want, doctors to prescribe the medications that their patients need,” Breyer wrote. “In §841 prosecutions, then, it is the fact that the doctor issued an unauthorized prescription that renders his or her conduct wrongful, not the fact of the dispensation itself. In other words, authorization plays a ‘crucial’ role in separating innocent conduct—and, in the case of doctors, socially beneficial conduct—from wrongful conduct.”

Breyer was joined in the opinion by Chief Justice John Roberts as well as Justices Sonia Sotomayor, Elena Kagan, Neil Gorsuch and Brett Kavanaugh.

Prosecutors alleged that the two doctors — Xiulu Ruan and Shakeel Kahn, who were tried separately — ran lucrative “pill mills,” flooding their patients with prescriptions for fentanyl and other serious pain management medications.

Ruan and Khan were convicted by juries and each received sentences of more than 20 years in prison. They challenged their convictions, arguing that the instructions the juries received risked criminalizing standard medical and prescription practices.