The second largest epidemiological study of LSD’s contribution to mental illness was conducted in the early 1960s by Dr. Sidney Cohen, a premier researcher of LSD studies and former Chief of Psychomatic Medicine at Veterans Administration Hospital in Los Angeles. Dr. Cohen reviewed a compilation of therapy studies including 44 psychiatric doctors representing almost 5000 patients and 25,000 uses of LSD or mescaline ingested in controlled psychotherapy sessions. He found the risk of psychosis lasting longer than 48 hours to be less than one half of one percent.⁸  

A larger study performed a decade later by Nicolas Malleson involving 49,500 psychedelic-assisted therapy sessions revealed like results of less than one percent.⁹  Similar results arose again in 1998 when Dr. John Halpern of Harvard University conducted a comprehensive review of 34 years of psychedelic research and concluded, “At present the literature tentatively suggests that there are few, if any, long term neuropsychological deficits attributable to long-term use.”¹⁰  In addition, the U.S. Army, which began extensive investigation into the effects of LSD on its soldiers in 1955, concluded,

There has not been a single case of residual ill effect. Study of the prolific scientific literature on LSD-25 and personal communication between U.S. Army Chemical Corps personnel and other researchers in this field have failed to disclose an authentic instance of irreversible change being produced in normal humans by the drug.¹¹

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