Take LSD seven times and you are legally insane.

The usual form is the one Snopes checked: "A person who has taken LSD more than seven times is automatically deemed legally insane."[1] The count changes with the teller. "The proposed number of times varies but is usually under ten", Erowid found, and Snopes agrees that "Seven appears to be the most common".[2][1]

The written record starts late. The earliest "documentation we have found for this myth is a series of Usenet posts from 1990", Erowid wrote, but people remember it from much earlier: "Rick Doblin of MAPS reports that he remembers hearing a variant of this myth around 1970".[2] Erowid's estimate is that "it seems this myth has its roots somewhere between 1967 and 1975", the years of the first LSD scare.[2]

There is no such law. Snopes found that "no state in the U.S. has such a law on its books".[1] The phrase itself means nothing in court: "There is no legal definition of the phrase", and past drug use on its own "does not qualify as a mental disorder for the purposes of an insanity plea".[2]

The nearest thing to a test in a courtroom went the other way. At the Manson trial the defence attacked Linda Kasabian's evidence over her LSD use, and Erowid notes: "Despite this attempt to discredit a witness for her admitted fifty LSD trips", "Manson and his cohorts were convicted and sentenced to death (later converted to life in prison) for the murders, based substantially on Kasabian's testimony".[2] Snopes's conclusion: "No specific number of LSD trips renders a user insane, in either a legal or psychiatric sense."[1]