The second study, to be published in the British Journal of Psychiatry, found magic mushrooms helped 10 patients vividly recall positive memories and feel generally happier in the following days.
Prof Nutt said: "This drug has such a fundamental affect on the brain we should be trying to understand why."
Professor Nick Craddock, from Cardiff University, said much more was needed to convince psychiatrists that psilocybin was safe, effective and acceptable.
Psychedelic drugs: A history
* The man regarded as the father of psychedelic drugs, Albert Hofmann, a Swiss chemist, first discovered LSD in 1938 and went on to isolate the active chemical in magic mushrooms, psilocybin, and several other naturally occurring hallucinogens.
* In the 1950s and 1960s, researchers carried out studies on around 40,000 patients and published 1000 papers on psychedelic drug-assisted psychotherapy. Such work was brought to an abrupt halt when the drugs were banned, making research virtually impossible.
* In 2010, American researchers published the first-ever clinical trial into the effects of MDMA, the active ingredient of ecstasy pills, found it to be safe and effective in treating patients with post traumatic stress disorder in conjunction with psychotherapy. Prof Nutt is hoping to replicate the study on UK veterans suffering from PTSD who have not been helped by conventional therapies.
* The therapeutic benefits of cannabis for patients with multiple sclerosis continue to be studied, 11 years after the active chemical compound, cannabinoid tetrahydrocannabinol (THC), was first shown to reduce muscle tremor and spasticity by scientists at University College London.
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