The history of THC

THC comes from the cannabis plant, which makes it in the resin glands of its flowers, and it was first identified in 1964 by the chemists Y. Gaoni and Raphael Mechoulam in Israel. People had used cannabis for thousands of years before that, but nobody knew which compound caused its effects. Their discovery led to the finding of the body's own cannabinoid receptors in the late 1980s and 1990s.

Where does THC come from?

THC is made by the cannabis plant itself. The enzyme THCA synthase produces THCA, the acid form of THC, inside the glandular trichomes, the small resin glands on the flowers and leaves (Sirikantaramas 2005). In the plant, the acid forms are the main cannabinoids. Heat removes a carbon dioxide group from THCA and turns it into THC, a step called decarboxylation (Wang 2016). Read more in What is THCA?

How much THC a plant makes depends on its genetics. Hemp varieties are bred to make very little, while drug varieties make a lot. A 2021 genome study found that genes involved in making the two main competing cannabinoids had lost their function in different lines during selection for fibre or for psychoactive properties (Ren 2021). Our guide cannabis, marijuana and hemp explains the difference.

When did people first use cannabis?

Plant remains show that cannabis was present near the Altai Mountains in Central Asia about 12,000 years ago, and its seeds travelled with nomadic peoples after the last ice age (Crocq 2020). A study of 110 cannabis genomes from around the world concluded that the plant was first domesticated in East Asia in early Neolithic times, and that today's hemp and drug varieties descend from a gene pool still found in China (Ren 2021). For most of that history, cannabis was grown mainly for fibre, rope, nets and seeds; see the history of hemp.

When was cannabis first used for its effects?

Some of the earliest directly dated evidence comes from wooden braziers found in tombs at the Jirzankal Cemetery in the eastern Pamir mountains in western China. Chemical residues show that cannabis was burned there during funeral ceremonies around 500 BCE, and that the plants produced high levels of psychoactive compounds (Ren 2019). In a grave in the same region from around 750 BCE, archaeologists found almost 800 g of cannabis with a high THC content, buried with a man of high status (Crocq 2020).

How was cannabis used in the ancient world?

The written record is patchy, and some famous claims are later legends:

• China: tradition credits the mythical emperor Shen Nong, around 2000 BCE, with discovering medicinal plants including cannabis. The herbal compendium attributed to him was written much later, during the Han dynasty (221 BCE–220 CE), so the early date is probably exaggerated (Crocq 2020).
• Egypt and Mesopotamia: the Ebers papyrus, written in about 1500 BCE, mentions cannabis applied to the skin for inflammation, and Assyrian clay tablets also refer to it (Crocq 2020).
• India: cannabis has been used for thousands of years in Ayurvedic medicine and had a religious role (Crocq 2020).
• Greece and Rome: Herodotus (5th century BCE) described Scythians throwing hemp seeds on red-hot stones and being "delighted" by the smoke. Pliny the Elder and Dioscorides described the root for inflammation, and Galen wrote that small cakes with cannabis were served as dessert in Italy (Crocq 2020).

How did cannabis reach Western medicine?

Europe rediscovered cannabis in the 19th century, partly through translations of Arabic texts by scholars such as Silvestre de Sacy. The Irish physician William Brooke O'Shaughnessy published his observations on cannabis in Calcutta in 1839. He noted that its intoxicating effect was then almost unknown in Europe, apart from a few young people in Marseille who had tried hashish brought back by Napoleon's soldiers from Egypt. The French physician Jacques-Joseph Moreau de Tours experimented with hashish himself and described its effects on the mind (Crocq 2020).

Cannabis tinctures were widely prescribed in the late 19th century, but doctors had a problem: the active ingredient was unknown, so the strength of the plant material varied from batch to batch, and doses were unpredictable. In the 20th century, medical use fell as other medicines replaced it and legal restrictions grew (Crocq 2020).

When was THC discovered?

Chemists had been trying to find the active compound of cannabis for decades. Cannabinol (CBN) was reported in 1899 (Wood 1899), and CBD was isolated in 1940 (Adams 1940), but neither explained the high.

The breakthrough came in Israel. Raphael Mechoulam, working at the Weizmann Institute, obtained 5 kg of seized cannabis from the police, separated its compounds and identified the one that was psychoactive (Crocq 2020). In 1964, he and his colleague Y. Gaoni published the isolation, structure and partial synthesis of this "active constituent of hashish": delta-9-THC (Gaoni and Mechoulam 1964). The year before, Mechoulam and Yuval Shvo had described the structure of CBD (Mechoulam 1963).

How did THC lead to the endocannabinoid system?

Knowing THC's structure allowed researchers to ask where in the body it acts. That question led to a whole signalling system:

• 1988–1990: a first cannabinoid receptor, CB1, was characterised in rat and human brain (Crocq 2020) and its gene was cloned in 1990 (Matsuda 1990).
• 1992: the first endocannabinoid, a substance the body makes itself, was isolated from the brain (Devane 1992). It was named anandamide, from the Sanskrit word ānanda, meaning bliss.
• 1993: a second receptor, CB2, was found outside the brain, mainly linked to the immune system (Munro 1993).

Together with a second endocannabinoid, 2-AG, these make up what is now called the endocannabinoid system (Crocq 2020). Read more in the endocannabinoid system explained and 2-AG and anandamide.

How has the law on THC changed?

Restrictions grew through the 20th century. The United States passed the Marihuana Tax Act in 1937, and the 1961 UN Single Convention on Narcotic Drugs placed cannabis in its strictest regime, Schedule IV, alongside heroin (Crocq 2020). THC itself is controlled under the 1971 Convention on Psychotropic Substances, where delta-9-THC is listed under its medicine name, dronabinol (UNODC 2020).

On 2 December 2020, following a WHO recommendation, the UN Commission on Narcotic Drugs voted 27 to 25 to remove cannabis and cannabis resin from Schedule IV. They remain in Schedule I and so stay under full international control. The commission rejected a proposed footnote that would have exempted preparations with mainly CBD and no more than 0.2% THC (UNODC 2020).

National rules on THC differ widely and change often, so check the current law where you live. Our country guides cover the rules for CBD, for example in Germany, France and Spain. Switzerland's study of regulated THC sales is described in THC gummies in Switzerland.

What do we know about THC today?

More than 60 years after its discovery, THC is one of the most studied plant compounds. It is the main cause of the cannabis high. In a controlled trial, 10 mg of vaporised THC impaired memory and caused short-lived psychotic symptoms in healthy volunteers (Englund 2023), and use while the brain is still developing carries a risk of psychosis and cognitive problems (Crocq 2020). For what THC does and how it differs from CBD, read What is THC? and CBD vs THC. For other intoxicating cannabinoids, see which cannabinoids are psychoactive?