Why 420? The true story of cannabis's code number

In North American slang, 420 means cannabis. The number is read "four-twenty", the way a time is said in English, and it is written three ways: 420 for the code word itself, 4:20 for the time of day, that is 16:20, and 4/20 for the date of April 20, since American notation puts the month before the day. According to the AP, the Oxford English Dictionary added it in 2017 after examining the Waldos' documents. Two uses grew out of it, covered below: the "420 friendly" label and the number slipped into a bio, a username or an ad. The same number also names the year 420 and a class of sailing dinghy; this article covers only the cannabis code.

No other account is as well supported, since its authors kept evidence. In 1971, five students at San Rafael High School, north of San Francisco, formed a group: Steve Capper, Dave Reddix, Larry Schwartz, Jeffrey Noel and Mark Gravich. According to Time magazine, they called themselves the "Waldos" because they gathered by a wall at the school.

Once after-school activities were over, they met at 4:20 pm by the statue of the chemist Louis Pasteur. They wanted to find a cannabis patch grown by a member of the Coast Guard near the Point Reyes peninsula, using a hand-drawn map that had come their way. As the AP report carried by CBS San Francisco explains, they never found it. The same report gives one reason for speaking in code: Jeffrey Noel's father was a narcotics agent for the California Department of Justice.

Their password, "420 Louie" in the Waldos' own spelling (the AP writes "420 Louis"), was soon cut down to "420" and outlived the treasure hunt, according to the AP. Time says they used "420" among themselves as a code word for cannabis. As proof that they came first, the Waldos kept items from the period, set out below.

The Waldos keep documents from the 1970s in which "420" already appears, in a safe deposit box rented at a San Francisco bank. The AP describes some of them, and the lexicographers of the Oxford English Dictionary examined their records before crediting them with the word. The rest is known only from the description the Waldos give on their own website. The table keeps these two levels of evidence apart.

A detail noted by the AP: the bank stands at 420 Montgomery Street.

The Waldos also have a commercial interest in the number. Their website adds the ™ symbol to several of their phrases and includes a shop. In 2018, the AP noted that they held none of the dozens of 420-related trademarks issued to other companies, that a nearby brewery was releasing a beer named after them, and that they had just struck a first deal with an Oakland cannabis company. That interest does not make their story false. It explains why the scrutiny of the press and of the Oxford English Dictionary carries more weight than their own claims.

The code left the school through the Grateful Dead's circle, and the two main sources do not describe the link in the same way. According to Time, Dave Reddix's brother helped him become a roadie for Phil Lesh, the band's bass player. The AP instead describes an older brother who was friends with Lesh, through whom the group got backstage at concerts, where the road crew picked up the code. Through that connection the number reached the musicians and their fans, the "Deadheads".

Things changed at the end of 1990. On 28 December, Deadheads in Oakland handed out a flyer that called on people to smoke "420" at 4:20 pm on April 20, according to Time. Steve Bloom, a High Times reporter at the time, got hold of one. The magazine printed it in 1991 and went on using the number; in 1998, again according to Time, it recognised the Waldos as the inventors of the code. Since American notation writes April 20 as 4/20, a time of day turned into a yearly date.

The AP places the take-off a little earlier, in the late 1980s, with flyers handed out at the band's concerts. Those flyers presented 420 as the California police code for cannabis use in progress. The explanation was false, yet it travelled with the number and, according to the AP, it still circulates.

The number even made its way into California law. The bill creating the state's medical cannabis programme, approved by the governor on 12 October 2003, is Senate Bill 420.

Outside France, April 20 has become a day of public gatherings. In the United States, activists and enthusiasts meet on that day in many cities, according to a study published in 2018 in JAMA Internal Medicine, and yearly festivals take place elsewhere, as in Vancouver, Canada, according to a study published in 2020 in the Emergency Medicine Journal. The legal status of cannabis differs from one country to another; our article on cannabis legalisation around the world compares the models in force.

Ten dated milestones are enough to follow the number, from San Rafael High School to current French law. Each row points to the source cited in this article.

No competing explanation shows 420 being used as a code before 1971. Most are coincidences noticed after the fact; the police-code story was already printed on the very flyers that made the number known. Time dismisses the first three in the table; the others can be checked against the texts or rest on no document at all.

Set against the Waldos' evidence, none of these leads holds. Only the high-school version is backed by documents from the period.

"420 friendly" signals that a person, a home or a place is fine with cannabis. The phrase spread along with the code in the United States: in 2018, the AP noted hotels and tour companies describing themselves that way, and dating sites where profiles called themselves "420 compatible". In a username, a bio or a message, "420" is only a nod to cannabis, with no other hidden code.

In France, this label has no legal value and makes no use legal: use remains a criminal offence under article L3421-1 of the Public Health Code. An ad or a post that presents cannabis use in a favourable light can also fall under article L3421-4, which punishes both incitement to use and that favourable presentation, as Drogues Info Service points out. The author's intent, the form and the context decide case by case. The rules on how CBD shops may communicate are set out in our article on CBD advertising in France. As for CBD products, a "420" mention changes nothing about their status, which depends, among other things, on a Δ⁹-THC content of no more than 0.30%.

Social media and campaign groups in France sometimes take up April 20, which changes nothing legally. Cannabis use is a criminal offence, punished under article L3421-1 of the Public Health Code by 1 year in prison and a €3,750 fine. Since 20 August 2026, the fixed fine that can replace prosecution is €500. It drops to €400 if paid quickly and rises to €1,000 when increased for late payment.

Marking the date is not banned in itself, and a gathering follows the ordinary rules on public events. Incitement to drug use is another matter: article L3421-4 punishes it with 5 years in prison and a €75,000 fine, even when it has no effect. That text can apply to a poster or a promotion inviting people to smoke cannabis on April 20.

Our article on French cannabis law sets out the penalties with figures.

The American studies do not agree. In 2018, Staples and Redelmeier counted, from 1992 to 2016, the drivers involved in a fatal crash between 4:20 pm and 11:59 pm (16:20 and 23:59) on April 20, then during the same hours one week before and one week after. They found 1,369 on April 20 against 2,453 across the control days, a relative risk of 1.12 with a 95% confidence interval of 1.05 to 1.19, according to their research letter in JAMA Internal Medicine. A letter published in the same journal in November 2018 re-examined that result.

In 2019, Harper and Palayew, two epidemiologists at McGill University, went back over the data from 1975 to 2016 in Injury Prevention. For the period 1992-2016 they found a ratio of 1.12 against the days one week before and after, but with an interval of 0.97 to 1.28, which does not rule out no effect at all. The ratio falls to 1.05 against the days one and two weeks either side, and to 0.98 against all other days of the year. They conclude that there is little evidence of a population-wide effect of April 20 on the number of drivers involved in fatal crashes.

In Vancouver, a University of British Columbia team that included John Staples, first author of the 2018 study, counted emergency department visits at six hospitals between 4:20 pm and 11:59 pm, from 2009 to 2018, in a study published in the Emergency Medicine Journal in 2020. The rise in visits for all causes was not significant, with an adjusted relative risk of 1.06. In subgroups defined in advance, visits for substance misuse were multiplied by 5 and visits for intoxication by 10, and the hospital closest to the festival recorded 17% more visits.

In France, the risk of cannabis at the wheel is established regardless of the date. The ActuSAM study, carried out by Ifsttar on the fatal crashes of 2011 and published by the national road safety observatory (ONISR), estimates that a driver who tests positive for cannabis multiplies by 1.65 the risk of being responsible for a fatal crash, and by 29 when also under the influence of alcohol. More than one in two drivers responsible for a fatal crash under cannabis were also under the influence of alcohol, and 4.2% of fatal crashes would be avoided if no driver tested positive for cannabis.

Driving after using drugs is punished by 3 years in prison and a €9,000 fine, and by 5 years and €15,000 when alcohol is added, according to page F2886 of Service-public. The same page provides for the loss of 6 licence points, or 9 with alcohol, and a licence suspension of up to 5 years. The ban applies whatever the amount consumed, on April 20 as on any other day. A full-spectrum CBD product can also make a saliva test positive, as our guide to saliva testing explains, together with our tool on CBD detection.

CBD comes from the same plant and has a legal regime of its own. A hemp product may be sold if it contains no more than 0.30% Δ⁹-THC, the limit set by the order of 30 December 2021. On 29 December 2022, the Conseil d'État annulled the ban on flowers and leaves contained in that order. The same rules apply every day of the year, April 20 included. What the law says about CBD itself is covered in is CBD a drug, and the plant's history in France in the history of hemp in France and Europe.

Two more points hold throughout the year. Apart from Epidyolex, prescribed for certain forms of epilepsy, CBD is not a medicine. A full-spectrum product can also make a saliva test positive, as our article on CBD and driving explains. The difference between the two plants is covered in hemp, cannabis, marijuana, and the basics in what is CBD.

We sort each fact by the kind of proof behind it: a public document, such as a law, a published study or the 1939 text; an item described by the press or examined by the Oxford English Dictionary; or a plain statement by the Waldos on their website. When two sources contradict each other, we say so. Time dates the Oakland flyer to 28 December 1990, while the AP places flyers as early as the late 1980s. Time makes Dave Reddix a roadie for Phil Lesh, while the AP describes a brother who was friends with the bass player. We could not read the 1991 issue of High Times that reprinted the flyer, as it is only available after signing in to the magazine's online archive. Sources checked on 20 October 2026; our approach is described on the methodology page.