(Reuters) - A U.S. appeals court on Friday revived claims that the National Football League was negligent in allowing teams to use prescription opioids and other drugs to dull players' pain and return them to the field in order to maximize profits.

A unanimous three-judge panel of the 9th U.S. Circuit Court of Appeals said a group of retired players led by Hall of Famer Richard Dent had adequately backed up their argument that by overseeing the teams' administration of drugs to players, the league also took on the duty of ensuring their safety.

"Despite the NFL's one-step-removed relationship to the players, it was within the NFL's control to promulgate rules or guidelines that could improve safety for players across the league," Circuit Judge Richard Tallman wrote.

An NFL spokesman and Patrik Shah of Akin Gump Strauss Hauer & Feld, who represents the league, did not immediately respond to requests for comment, nor did the players' lawyers at Silverman Thompson Slutkin & White.

Dent and seven other retired players sued the NFL in 2014, claiming the league spearheaded a "return to play" policy in which injured players were given painkillers and sent back into games, leading to further injuries.

The players accused the league of negligence, negligent hiring and retention, fraud, and other claims on behalf of a proposed class of athletes who had played in the NFL since 1969 and were administered drugs.

U.S. District Judge William Alsup in San Francisco dismissed the case in 2015. He said that because the players had entered into collective bargaining agreements with their teams, federal labor law precluded them from bringing the claims to court.

The 9th Circuit in 2018 reversed, ruling that the claims were not preempted because the NFL was not a party to the players' bargaining agreements.

On remand, the plaintiffs filed an amended complaint in which they abandoned all but their negligence claims.

U.S. District Judge William Alsup last year said the league could not be held liable for negligence because it was not directly involved in distributing or administering medications.

The players appealed, and the 9th Circuit on Friday agreed with Alsup that the plaintiffs could not back up a per-se negligence claim, since they had not alleged that the NFL violated any specific laws or contractual obligations.

But the players could succeed on their alternate theory that the NFL voluntarily took on the duty of overseeing the administration of drugs by regulating their use by teams.

The league since 1973 has required teams and their doctors to submit reports on the administration of medications, and for nearly 30 years has audited teams' drug use and compliance with federal drug laws, the court said.

The panel sent the case back to Alsup to decide whether the "voluntary undertaking" claim was preempted by federal labor law. Alsup had not considered that claim in his 2016 decision dismissing the case.

The panel included Tallman and Circuit Judges Jay Bybee and N. Randy Smith.

The case is Dent v. NFL, 9th U.S. Circuit Court of Appeals, No. 19-16017.

For the plaintiffs: William Sinclair of Silverman Thompson Slutkin & White

For the NFL: Pratik Shah of Akin Gump Strauss Hauer & Feld