Dr Rupert McShane, who has led a trial in Oxford since 2011, says ketamine can work on patients like Helen who have depression "where nothing has helped before".

He adds there should be a national registry for those who prescribe the treatment to monitor the results and avoid misuse of the Class B substance.

Commenting on the Lancet Psychiatry paper, Prof Allan Young, from the The Royal College of Psychiatrists, said there were still "significant gaps" in knowledge about ketamine's use.