In an enormous glasshouse a few hours from London, there’s a powerful, unmistakable smell in the air: it’s the one that seems to cling to some surly teenagers and drifts around on the breeze at pop festivals. Here, 30,000 cannabis plants sway gently beneath giant fans and immensely bright lights. Only the remarkable uniformity of the plants – and the people walking round in lab coats – tells you the place isn’t some drug lord’s illicit cannabis factory.

This is the only research facility in the UK licensed to grow cannabis on a vast commercial scale. Here, Dr David Potter has overseen the production of nearly 2m cannabis plants, mostly for medical research or the production of the cannabis-based multiple sclerosis drug Sativex. He is director of botany and cultivation for GW Pharmaceuticals, a company that is exploring how cannabis could help treat a range of illnesses ranging from epilepsy to cancer.

Recently Potter and GW’s team have turned their attention to developing a cannabis-based treatment for psychosis and related illnesses such as schizophrenia. For a drug that is widely seen as a trigger for acute psychotic illness in young users, this at first sounds preposterous. But, as Potter explains, the cannabis plant is much more than just a psychedelic weed.

“The most well-known ingredient in cannabis that gets people high is THC [or tetrahydrocannabinol],” says Potter, who often travels to give talks in London carrying a suspicious-smelling suitcase of the plants. “But THC is just one of dozens of potentially useful cannabinoids in the plant.”

Cannabinoids are chemicals that act on the brain’s cannabinoid receptors, part of a system that regulates a variety of physiological processes including pain sensation, mood, memory and appetite.

In high doses, THC can induce temporary schizophrenia-like psychotic symptoms such as paranoia, delusions, anxiety and hallucinations. Yet cannabis also contains a cannabinoid known as CBD (or cannabidiol), which appears to have almost the exact opposite effect.

Purified CBD has been shown to have antipsychotic and anti-anxiety effects, and can lessen the psychotic symptoms normally experienced by people given high doses of THC. Research by University College London also suggests that people who smoke cannabis rich in CBD are less likely to experience “schizophrenia-like symptoms” than those who smoke cannabis containing only THC.

Unfortunately for the mental health of many young cannabis users, the chemical profile of the drug has changed drastically over the past three decades. Not only does modern cannabis contain more than twice as much THC as it did in the 1960s, it also now contains hardly any of the “neuroprotective” cannabinoid CBD.

Potter has analysed countless samples of street cannabis on behalf of the Home Office and the police, in parallel to his work cultivating the plant for medical research. With this “library” of cannabis samples and seeds stretching back decades, he has been able to track exactly how cannabis has changed.

The origins of modern cannabis can be traced to California in the late 1970s, when professional breeders began to select the most potent, THC-rich plants for the first time. “Until then, most cannabis came in the form of hashish resin, made of mixed populations of plants from parts of Asia, Africa, and the Caribbean, containing varying amounts of both CBD and THC,” says Potter.

In the quest to make ever-stronger cannabis, growers may have inadvertently bred out a protective chemical

Due to the complex genetics of the cannabis plant, growers selectively breeding THC-rich plants were also selecting against the gene that produces CBD. In other words, in the quest to make ever stronger cannabis, illicit growers may have inadvertently bred out a chemical that protected the mental health of users in the past.

“When skunk was created the people doing it had no idea they were altering the ratios of CBD and THC – they just kept breeding the plants that gave the strongest high and threw the rest away.”

By 1984, a variety of cannabis known as “skunk#1” arrived in Europe. Named after its pungent smell, it contained 15% THC instead of the 1-8% found in older varieties.

Pot smokers in the UK never looked back and skunk is now by far the most prevalent type of cannabis sold illegally here. By 2004-5, Potter could find hardly any traces of CBD at all in a batch of 500 seized cannabis samples given to him for analysis by the Home Office.

The exact effect this change has had on the mental health of cannabis users is hard to say – the link between cannabis and schizophrenia remains complex and controversial. Researchers have struggled to prove whether cannabis causes psychosis, or whether people predisposed to psychosis are just more likely to smoke cannabis. The best evidence currently suggests that in people who are genetically at risk of schizophrenia, regular cannabis use doubles their risk of experiencing psychotic symptoms. But no long-term studies of people with schizophrenia have chemically analysed the type of cannabis the subjects were smoking.

Potter has seen the extreme effects of both substance abuse and schizophrenia in his role as a magistrate for his local court. He recounts a recent case where a seemingly pleasant teenager, suffering from acute and sudden psychotic illness, had turned to alcohol and become violent.

“He ended up smashing someone’s teeth out for not really any reason at all,” says Potter. The defendant, now diagnosed and medicated, will still be found guilty despite his temporary insanity. “In court it just struck me what a nice chap he was.”

The need for new antipsychotic drugs is pressing. Around 0.5% of the population are thought to suffer from schizophrenia – as many as 20-30 million at any one time globally – and around a third of those do not respond to antipsychotics.

Existing medication does little to treat the other disabling symptoms of the disease – chronic inflammation, low mood, anxiety, and cognitive impairment – and often has worrying side-effects including weight gain, involuntary movements and drowsiness.

All antipsychotics act primarily by altering the brain’s production of dopamine, and the fact that CBD acts in a different way could be helpful for that third of people with schizophrenia who don’t respond to existing drugs. Research suggests CBD, with its anti-inflammatory and anti-anxiety effect, could also help treat schizophrenia’s broader symptoms, not just the psychosis.

Skunk, which is now by far the most prevalent type of cannabis sold illegally in the UK. Photograph: Michel Porro/Getty Images

Results from double-blind, phase II clinical trials for CBD as a schizophrenia treatment will report next year. For now hopes rest on a small trial involving 39 patients with schizophrenia, 19 of whom were given the antipsychotic drug amisulpride, the rest CBD. At the end of the four-week trial, both groups showed significant improvement in their symptoms, but the CBD cohort reported far fewer side effects.

Potter and his team remain cautious, as it is not yet fully understood how CBD actually works – it is simplistic to say CBD just does the opposite of the psychosis-inducing THC. One theory is that CBD boosts the activity of other cannabinoids produced naturally by the brain.

Irrespective of the results of GW’s clinical trials, research from around the world on CBD seems to have kick-started a movement among some cannabis users to switch to CBD-rich varieties in the hope of a safer, more mellow drug. Supposedly CBD-rich cannabis products are now being sold by “therapeutic” retailers from Amsterdam to San Francisco – and if more research finds CBD-rich cannabis really is significantly safer than skunk, it could strengthen the case for the legalisation and regulation of cannabis products.

Potter is passionate about cannabis, but he is not a politician. He does not wish to enter the debate about legalisation, and doesn’t advocate any type of smoking because of its dire health implications. All of the medicines GW develops are administered in traditional routes such as throat sprays or liquids, and none get users high.

He now boasts that his plants have as much CBD as there is THC in the strongest skunk, after years of selective breeding in the opposite direction by illegal producers.

It has taken him more than a decade to get to a point where he can grow both CBD- and THC-rich cannabis of consistent strength on a wide scale. When he tried to find CBD-rich varieties with which to conduct medical research in the late 90s, 97% of commercially available seedlings created THC-dominant plants.

“Seeds for CBD-rich plants were extremely difficult to find,” he recounts. “It took me to all sorts of places, including one quite strange shop in Amsterdam.

“We can now produce plants of up to 15% CBD, way more than you would find naturally. But it’s our productivity that is really much higher than illegal growers,” he says proudly.

The surreal, secret glasshouse in which Potter works understandably involved extensive discussions around security before the government would grant a licence. So far, says Potter, no attempts have been made on the bounty inside.

Half of the tens of thousands of plants growing here at any one time are CBD-rich, half THC-rich. The two varieties are harvested; the cannabinoids extracted and blended in different ratios for different therapeutic effects.

It’s unlikely any intruders would be able to distinguish between the two varieties anyway – half would be utterly useless to recreational users. But thanks to the promise CBD has shown in a range of therapies under investigation, Potter’s CBD-rich crop is potentially much more valuable than the THC crop.

He admits it is “a thrill” to work with a plant with so many cultural and spiritual connections. He describes the rare, original skunk#1 plants from California, which now reside in GW’s plant library, as having “significant social history” in the way they changed cannabis and its image for ever. Perhaps his latest crop, rich in CBD and low in THC, will be the next cannabis plant of significant importance to society.

There’s one final question you have to ask a man who has overseen the production of millions of cannabis plants and who has samples of some the strongest skunk ever seized in the UK in his office. Has he ever got “high off his own supply”, as they say?

“I never have, and it’s probably best to keep that genie in the bottle,” says Potter. “If I find I like it and I’m surrounded by it all day it might be a bit of a problem.”

MEDICINAL CANNABIS: Current therapeutic usage

Cannabinoids from the cannabis plant are being explored for a range of therapeutic uses. Medicinal cannabis extracts are currently prescribed in a number of countries to alleviate pain, treat muscle spasticity, and reduce nausea during chemotherapy.

CBD (cannabidiol) is being investigated as a potential treatment for epilepsy, diabetes, appetite-loss, a range of inflammatory diseases such as arthritis, and psychosis and schizophrenia. The compound has also been found to be a potent antioxidant and even appears to inhibit cancer cell growth in certain rare cancers.

The fact that chemicals produced by cannabis interact with key receptors in the body is purely a coincidence, it is believed. Cannabinoids are thought to be merely a defensive mechanism to protect the plant against predators and harsh environmental conditions.

A mixture of dozens of cannabinoids and bitter-tasting oils are produced in special structures called trichomes, tiny membrane-bound globules which are mostly clustered around the flowers of the plant.

When an invading insect breaks the delicate membrane of the globule, volatile oils evaporate off, like in a solvent-based adhesive, and the insect is glued to the plant in seconds.

THC, therefore, much revered for its psychedelic effects, is nothing more than an ingredient in nature’s Loctite, it seems. As are dozens of other potentially useful compounds.