It's dramatic and it's exciting, and it is a novel mechanism. But it's not about to become a routine treatment," Rupert McShane, a consultant psychiatrist and researcher at Oxford University who led the study, told reporters.
He said the discovery that ketamine worked, even for a short period, had been enough to give new hope to some of the patients in the study - many of whom had in the past considered suicide.
"We've seen remarkable changes in people who've had severe depression for many years that no other treatment has touched," McShane said. "It's very moving to witness."
Although many of them relapsed within a day or two, almost a third of them felt a benefit which lasted at least three weeks, he said, and 15 percent did not relapse for more than two months.
"We now need to build up clinical experience with ketamine in a small number of carefully monitored patients," McShane said. "By trying different infusion regimes and adding other licensed drugs, we hope to find simple ways to prolong its dramatic effect."
Ketamine is a licensed medical drug, widely used as an anesthetic and to relieve pain. But it is also used as a recreational drug and can lead some people into drug abuse.
McShane said that the doses used were very different. On the street, users often take several grams a day and can suffer severe bladder problems and impaired brain function. The dose used in the study was no more than 80 milligrams - 80 thousandths of a gram - every week in a closely monitored hospital setting.
Several research teams around the world have been studying the potential for ketamine use in depression, since many patients with the psychiatric condition fail to respond to currently available antidepressants such as Prozac and Seroxat.
The U.S. drug giant Johnson & Johnson is developing an intranasal form of the drug, called esketamine, in mid-stage trials and has said its results so far have been promising.
Anglo-Swedish drugmaker AstraZeneca had also been conducting early-stage trials on a ketamine-like drug, but opted to discontinue its development after disappointing results.
In the Oxford study, 28 patients with severe and treatment-resistant depression were treated over three weeks and received either three or six ketamine infusions each lasting 40 minutes.
Memory tests were carried out a few days after the final infusion and patients reported their mood symptoms daily via text or email.
The results, published in Journal of Psychopharmacology, showed that three days after the last infusion, the depression scores had halved in 29 percent of the patients.
Some patients described feeling able to think freely for the first time in years, he said, adding: "For some, even a brief experience of response helps them to realize they can get better and this gives hope."