The original theory was that 420 was California police code or penal code for cannabis offenses. This later proved to be incorrect. In the penal code, 420 involves obstructing entry on public land, and the number stands for nothing in the police code. Likewise, the use of any drug, including cannabis, is 11350 in the California Health and Safety Code.
Another theory was that the cannabis plant contains 420 separate chemicals, per a handful of textbooks. Very few school textbooks get into detail about the chemical composition of the plant, and while a botany book might, the authors would likely know that the plant does not contain that many separate chemicals. Some even suggested that 420 refers to tea time in Holland, but even if the time was correct (it is not), it is still a stretch to suggest that tea time in Amsterdam inspired a code word 5,500 miles away in California.
Some point to the music world. In 1966, Bob Dylan led off Blonde on Blonde with “Rainy Day Women Nos. 12 & 35” featuring the main chorus line, “Everybody must get stoned.” Some math nerds pointed out that 12 x 35 equals 420, but if the song were the inspiration, 1235 would have been the more logical code number. The same can be said about the April 1966 release of The Who’s My Generation, with the U.S. album cover showing Big Ben at 4:22pm. The association is too much of a stretch.
Crosby, Stills, Nash and Young do have a song titled “4+20” but the lyrics have nothing to do with cannabis. Others have suggested that April 20 is the date of Bob Marley’s death, but he passed on May 11, 1981, after 420 had already appeared on flyers. Less popular theories tie 420 to the passing of Jim Morrison, Jimi Hendrix or Janis Joplin, though none of the 27 Club members (artists who passed away at age 27) died on April 20. Some also claimed the Grateful Dead headquarters were located at 420 Ashbury and that Jerry Garcia died at 4:20 in 1995. Regarding the latter, the time was 4:23 a.m., and 420 was already in circulation by 1995. As far as the address, the Dead headquarters were at 710 Ashbury, a number that dabbers will certainly find ironic.
Another theory is that 420 refers to Albert Hofmann’s famous psychedelic experience on April 16, 1943. Per his lab notes, the Swiss scientist took lysergic acid diethylamide (LSD) at 4:20pm as history’s first acid trip. Still, obscure notes regarding a different substance in an experiment that took place three decades earlier make the proposed connection sound more like a happy coincidence. There is also an 18th-century nursery rhyme called “Sing a Song of Sixpence” with the line "4 and 20 blackbirds baked in a pie,” and while this theory is fun, the old British rhyme has seemingly no connection to cannabis.