Medical groups are hailing a federal appeals court’s decision to grant a new trial to a Virginia doctor accused of drug trafficking for prescribing large quantities of narcotics to his patients.

The 4th Circuit Court of Appeals ruled Tuesday that jurors at the 2004 trial of Dr. William Hurwitz of McLean, Va., were improperly prevented from considering whether he acted in good faith. He was convicted on 50 counts and sentenced to 25 years in prison.

An appeals court judge, William Traxler Jr., wrote that the jury should have been told that Dr. Hurwitz committed no crime if he had a reasonable, good-faith belief that the prescriptions were appropriate medical care. “A doctor’s good faith in treating his patients is relevant to the jury’s determination of whether the doctor acted beyond the bounds of legitimate medical practice,” Judge Traxler said. “The district court effectively deprived the jury of the opportunity to consider Hurwitz’s defense.”

The case attracted the attention of physicians and patients’ advocates who said the tough punishment could discourage doctors from providing adequate medication to those with severe pain.

Prosecutors argued that Dr. Hurwitz wildly overprescribed pain-killing drugs such as OxyContin and Dilaudid, giving one patient prescriptions for more than half a million pills over a 40-month period. The government also said there was evidence the physician suspected that some of his patients were selling a portion of their prescribed medicines.

Defense attorneys contended that Dr. Hurwitz acted within professional standards and that patients suffering from long-term pain can require extremely high doses of narcotics because smaller doses often become ineffective over time. The defense acknowledged that Dr. Hurwitz had been disciplined by state medical boards, but said those agencies “were back in the Stone Age.”

A past president of the American Academy of Pain Medicine, Dr. Scott Fishman, said his group has not formally endorsed or rejected Dr. Hurwitz’s methods, but filed an amicus brief because the court proceedings put all doctors in jeopardy. “On the surface, this is a case that appears to be an issue around a doctor who is at the extreme of practice, if not over the line,” Dr. Fishman, a professor at the University of California at Davis, said. “It was about whether he got a fair trial. We felt if we didn’t stand up in this case, future legitimate appropriately prescribing physicians could be prosecuted as drug dealers. We’re trying to protect patients.”

A libertarian-oriented physicians group, the American Association of Physicians and Surgeons, offered a more robust defense of Dr. Hurwitz. “Billy Hurwitz is an accomplished physician at the cutting edge of medicine, trying to help his patients,” a lawyer for the group, Andrew Schlafly of Far Hills, N.J., said. “This guy published articles in medical journals. He had a medical approach.”

Mr. Schlafly said federal prosecutors overreached by using the criminal justice system to control doctors. “The federal government has no business mischaracterizing a doctor as a drug dealer and interfering in the states’ regulation of the practice of medicine,” the lawyer said.

Mr. Schlafly said the defense victory was remarkable because the 4th Circuit is considered the most pro-government of the federal appeals courts. “This was a rarity, really,” he said.

An attorney for Dr. Hurwitz, Lawrence Robbins, said he was “delighted” with the decision. He said overzealous prosecutors had “chilled the availability of legitimate and legal pain management.”

Prosecutors said through a spokesman that they were considering their options, which include asking the full bench of the 4th Circuit to rehear the case.

While the appeals court panel was unanimous in granting Dr. Hurwitz a new trial, they differed over precisely what jurors should have been told. Judge Traxler and another judge, Cameron Currie, favored telling jurors that the doctor’s belief he was acting appropriately had to be both in good faith and objectively reasonable. However, the third judge on the panel, Hiram Widener Jr., dissented, saying that made little sense. “The two terms are contradictory, it seems to me,” he wrote.