The Supreme Court on Tuesday grappled with the convictions of a pair of doctors who were found to have been running opioid "pill mills," a case where the justices are being asked to find the line between legitimate medical practice and criminal drug distribution that has contributed to a nationwide crisis.

The doctors, both sentenced to more than 20 years in prison, are challenging their convictions, arguing that medical professionals should not be tried as drug dealers when they believe they are prescribing medication for a legitimate medical purpose.

"If it is sufficient to find only that a doctor acted outside the usual course of practice without reference to the purpose of the prescription, then doctors can be convicted for failing to follow medical norms, even if they never prescribed for an illegitimate reason," Beau Brindley, an attorney representing one of the doctors, told the justices on Tuesday.

"This allows conviction of doctors who misapprehend the extent of their obligations, whether or not [they are] drug dealing as conventionally understood."

The arguments on Tuesday focused on close readings of the criminal law around controlled substances, which ban the distribution of those medications unless prescribed by a licensed doctor "acting in the usual course of his professional practice."

It's unclear how the court will rule in the case, and the justices appeared conflicted about where to draw the line between a doctor operating in the usual course of practice and one who is trafficking in narcotics. Some on the bench questioned the doctors' lawyers insistence that defendants should be protected by a defense that they are operating in good faith when prescribing the addictive, and often dangerous, medications.

"What if a doctor legitimately believes that legitimate medical practice encompasses giving people who are dependent on drugs the drugs they need to satisfy that dependency?" said Justice Samuel Alito. "That's what the doctor really thinks."

"The doctor has to be acquitted, in your view?" Alito asked.

Brindley responded, "If the jury believes that he's sincere, and his belief that that's a legitimate purpose, I think that is true. But I don't think that's very likely to occur when all the objective evidence comes in saying that's wrong."

Other justices seemed cautious about how to interpret what they see as unclear language in the laws and regulations around controlled substances and the implications that an imprecise ruling could have for the medical profession.

Justice Brett Kavanaugh suggested that regulations to provide more clarity might be needed.

"The 'legitimate medical purpose' is a very vague thing on which reasonable people can disagree," Kavanaugh said.

The arguments on Tuesday concerned a pair of cases involving two separate doctors, each facing decades in prison for their prolific prescriptions of opioids and other drugs.

Xiulu Ruan was convicted of unlawfully distributing controlled substances out of a medical clinic in Mobile, Ala., and sentenced to 21 years in prison. Prosecutors said that Ruan and a partner were some of the top opioid prescribers in the country, doling out more than 300,000 scripts between 2011 and 2015.

The other case concerns Shakeel Khan, a doctor who operated a cash-only clinic out of a small town in Arizona until local pharmacies stopped filling his prescriptions. Khan opened a new practice in Wyoming, where he was later tried, convicted and sentenced to 25 years in prison on distribution counts, including one that led to an overdose death.

Eric Feigin, a deputy solicitor general arguing on behalf of federal prosecutors who brought the cases, told the court that the prosecutions were straightforward and to accept the doctors' legal arguments would invite predatory practices to run rampant through the medical field.

"They really are asking this court to transform their DEA registrations, which are premised on the idea that they're actually practicing medicine, into licenses to ... violate the general rule that drug pushing is illegal," Feigin said.

"They want to be free of any obligation even to undertake any minimal effort to act like doctors when they prescribe dangerous, highly addictive and, in one case, lethal dosages of drugs, to trusting and vulnerable patients."

The Supreme Court will issue a decision by early July.

How the court decides the doctors' cases could have implications for the Justice Department's efforts to go after the pill mills that experts believe helped facilitate the opioid crisis.

If the court sides with the doctors, prosecutors may have a hard time bringing cases and securing convictions against licensed physicians who profited from supplying their patients with great quantities of the harmful substances.

Updated at 1:13 p.m.