Georges Lederman, a lawyer for Dr. Neumeister, said there may have been protocol violations, “but N.Y.U. has taken the position that those violations were more egregious than we believe they actually were.” The issues could have been easily remedied, he said, and noted that they did not cause the sponsor of the research, the pharmaceutical giant Pfizer, to shut it down.

Both the university and Mr. Lederman said there was no evidence that any study participant had been harmed.

Pfizer said that N.Y.U. was responsible for conducting the trial, and that the company had previously tested the same drug, known as an F.A.A.H. inhibitor, for osteoarthritic pain, without significant side effects. “The safety profile we observed does not preclude future development of our compound,” a Pfizer spokesman said by email.

Careful oversight is a centerpiece of ethical research practice, particularly when studies involve people taking an experimental drug. Earlier this year, six patients in a French trial of another experimental drug with similar, marijuana-like effects were hospitalized with severe neurological problems; one has reportedly died.

Study participants with mental disorders are especially susceptible to adverse reactions, experts said. “These are people who are more vulnerable to being exploited in the research process, and more vulnerable to things going wrong during the research, so you want extra vigilance,” said Elisa Hurley, the executive director of Public Responsibility in Medicine and Research, a national nonprofit devoted to promoting high research standards. “If someone in my family were in a situation like this, I would want to be sure that the institution was crossing its t’s and dotting its i’s.”

The study was an attempt to extend a small trial that Dr. Neumeister had done previously, suggesting that cannabis might relieve anxiety in some people with post-traumatic stress disorder. “We know very well that people with PTSD who use marijuana often experience more relief from their symptoms than they do from antidepressants and other psychiatric medications,” Dr. Neumeister said in a 2014 N.Y.U. news release after the first trial.