“These are exciting times, for sure,” said Dr. Todd Gould, an associate professor of psychiatry in the University of Maryland School of Medicine. “We have drugs that work rapidly to treat a very severe illness.” Dr. Gould was not involved in the Janssen study but has identified a metabolite, or ketamine breakdown product, that could be developed into another drug.

Experts with long experience in treating depression were encouraged by the news, but also chary. The effectiveness of the previous class of antidepressants such as Prozac and Paxil was vastly exaggerated when they came on the market. And the results of esketamine trials, which were paid for and carried out by Janssen, were mixed.

In each trial submitted, all patients were started on a new antidepressant drug, and given a course of esketamine treatment or a placebo. In one monthlong study, those on esketamine performed better statistically than those on placebo, reducing scores on a standard, 60-point depression scale by 21 points, compared to 17 points for placebo. But in two others trials, the drug did not statistically outperform placebo treatment. Historically, the F.D.A. has required that a drug succeed in two short-term trials before it is approved; the agency loosened its criteria for esketamine, opting instead to study relapse in people who did well on the drug.

In that trial, Janssen reported that only about one-quarter of subjects relapsed, compared to 45 percent of subjects who received the placebo spray. All the subjects had been given a diagnosis of treatment-resistant depression, or T.R.D., having previously failed multiple courses of drug treatment.

“We’ve had nothing new in 30 years,” said Steven Hollon, a professor of psychiatry and behavior sciences at Vanderbilt University. “So if this drug is an effective way to get a more rapid response in people who are treatment resistant, and we can use it safely, then it could be a godsend.”

Desperate for relief

One question that will need to be answered is how well esketamine performs in comparison to intravenous ketamine.

Theresa, 57, an adjunct professor of English in New York, who asked that her last name be omitted to protect her privacy, has lived much of her life with deep depression. She tried a course of I.V. generic ketamine last summer, at a local clinic, which typically entails a half-dozen infusions, given over a couple of weeks, for about $500 apiece, with follow-up “booster” treatments as needed.