The first UK study to give ketamine to severely depressed patients has found that it had dramatic positive effects on some long-standing sufferers who had not responded to other treatments.

The government announced in February that ketamine would be upgraded to a Class B banned substance in the face of evidence that it has caused physical and psychological harm to recreational users.

But researchers at Oxford Health NHS Foundation Trust and the University of Oxford observed that 29% of patients in a study, some of whom had suffered from depression for more than 20 years, experienced significant improvement in mood four to seven days after their final dose of the drug, with four of the 28 subjects them completely free from depression at that point.

Dr Rupert McShane, a consultant psychiatrist at Oxford Health and researcher in Oxford University's department of psychiatry, warned against self-medication, emphasising that patients in the trial were carefully monitored.

He said: "This really is dramatic for some people and it's the sort of thing that really makes it worth doing psychiatry … we've seen remarkable changes in people who've had severe depression for many years that no other treatment has touched.

"It's very moving to witness. Patients often comment that the flow of their thinking seems suddenly freer."

About one in 10 people in the UK suffer from some form of depression in their life and McShane estimates that 180,000 people suffer from moderate or severe depression that has not responded to other treatment.

The Oxford study, published in the Journal of Psychopharmacology on Thursday, involved 28 patients with treatment-resistant depression being given either three or six intravenous infusions of 0.5mg/kg of ketamine, each lasting 40 minutes, at Warneford hospital's ECT clinic in Oxford.

Four to seven days after the final infusion, eight of them had responded to the treatment, measured by their depression "score" falling by 50% or more, of whom four were in remission.

Among those who responded to ketamine, the duration of benefit varied widely, from 25 days to eight months, with the median 2.3 months.

Ketamine is seen by the researchers as a substitute for electroconvulsive therapy (ECT), which is sometimes used for people with treatment-resistant depression but carries risk of memory loss.

One of the reasons the drug is thought to work is that it has a direct impact on the subgenual anterior cingulate, the part of the brain where overactivity is seen in people with depression.

It works rapidly compared with some antidepressants, which can take 10 or more days to take effect.

One of the reasons ketamine was upgraded to Class B was because it has been associated with major bladder problems, but these were not experienced by the patients in the trial.

They were given about half the amount of a typical recreational dose, although heavy users of the drug, popular among clubbers, take several grammes a day.

Some patients experienced distorted perceptions and/or felt disconnected from their body but only during infusion. They did not feel euphoric nor suffer loss of memory.

Side effects experienced included anxiety during the infusion and vomiting and one patient fainted.

Episodes of suicidal behaviour occurred during the study – not unusual given the patients were severely depressed – but the researchers said suicidal ideas diminished overall.

"The main thing we are having to work on now is maintenance strategy to try to prolong the responses [to ketamine]," he said.

"It's a controversial area but there's no doubt that it's got potential."

He did not anticipate the reclassification of the drug causing any problems, noting that it is already widely used in relief of chronic pain in the UK.

The study team would like to hear from patients and others who are interested in this area. Please register this interest at https://www.surveymonkey.com/s/9RQP5KL

• This article was amended on 3 April 2014 because the original incorrectly said 5mg/kg ketamine was administered in the Oxford study. This has been corrected to say 0.5mg/kg.