Monroe said he had seen the toll the game took on several of his friends who had already retired, and he wanted to end his career before his body deteriorated further and he needed to take yet more pills to continue playing.

“I don’t want to have to continue to consume pills to do that; I don’t want to do that,” he said. “Anti-inflammatories or opioids, which I certainly don’t want to take, that is certainly the option to stay within the rules of the game.”

Monroe said he would continue to call for players to be allowed to use medical marijuana. He said he had been in talks with the N.F.L. Players Association, the players’ union. The league has not softened its prohibition against medical marijuana, though Commissioner Roger Goodell said that the league’s medical advisers would continue to study its uses and efficacy.

Monroe said that while he was now free to promote medical marijuana, he hoped current players would also raise their voices. Many of them want more research on the effects of medical marijuana but are afraid to say so publicly for fear of upsetting their employers, he said.

“I’ve had conversations with my teammates and have been in conversations with players, and at the very least, they believe more research is needed to find a better option,” Monroe said.

But, he added, “there is also a great amount of reluctance for people to jump out and do the same thing, and say they believe there are issues.”