Tests showed that the ketamine patients had significantly fewer depressive symptoms than those who received the placebo — beginning 40 minutes after the infusion and lasting three days. After that, the effect of the ketamine began to fade, and within two weeks those who had received the medicine had about the same symptoms of depression as those who had taken the placebo.

There were no serious side effects, although about 10 percent of both those receiving ketamine and those taking the placebo got a headache or nausea, or reported feeling woozy or lethargic after the infusions. The adverse effects associated only with ketamine included feeling strange, having a dry mouth, increased blood pressure or faster heartbeat. After about 80 minutes, there were no differences in side effects between the ketamine and placebo groups.

The scientists view these findings as highly significant, because the patients had failed an average of seven antidepressant trials, and more than half had failed to respond to electroconvulsive therapy. But they also emphasized the study’s limitations. The number of patients was small, all were late in the course of their illness, and all were also taking valproate or lithium, which may have affected the results. Finally, there was a small but insignificant placebo effect.

Dr. Dost Ongur, an assistant professor of psychiatry at Harvard, doubted that the experiment could be truly double-blinded — the vivid effects of injected ketamine can be easy to perceive, both by the patient and by the person giving the injection.

Still, he was impressed. “This is a solid piece of work that really has the potential for being a true advance,” he said. “There is real hope, there is real excitement around this, even though there is skepticism from some quarters.”