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Cambridge University Science Magazine
In 1996 Californians voted to introduce an untested medicinal drug into the public marketplace. Although the drug had not been through clinical trials, plenty of people had apparently tested it for themselves and were convinced of its effectiveness. I’m talking about cannabis (aka: marijuana, pot, weed), a drug that was illegal nearly everywhere else at the time.

This unprecedented public action was followed by the legalisation of cannabis for medical use in 23 US states and many European countries (not including the UK). Now the medical marijuana business is booming; cannabis is used to treat multiple sclerosis, epilepsy, rheumatoid arthritis, Parkinson’s disease and Alzheimer’s disease. In the UK, NHS patients with multiple sclerosis can be prescribed a cannabis-based medication called Salivex.

While the internet offers numerous anecdotes proclaiming the potential medicinal benefits of cannabis, scientific research into has been slow to keep up. This is partly due to the strict regulations controlling research involving illegal drugs. Famously, until recently researchers in the USA only had access to 1-2 strains of cannabis plants, strains that were much less potent than the cannabis available to buy on the street. Despite these limitations, clinical trials do generally support claims that cannabis helps relieve chronic pain.

"...clinical trials do generally support claims that cannabis helps relieve chronic pain."

To understand how and why the cannabis plant may act as a medicine, we need to consider the chemical compounds that make it up. Cannabis contains about 70 different compounds called cannabinoids, which are very similar to chemicals produced naturally inside our own bodies called endocannabinoids. They act as neurotransmitters, a group of chemicals which pass messages between cells in the central nervous system. The compounds in cannabis are so similar to endocannabinoids that they interact with cannabinoid receptors in the brain, affecting our feelings of pleasure, pain, mood and hunger. Two cannabinoids are particularly interesting for medical applications: ∆9-tetrahydrocannabinol, often called THC, and cannabidiol or CBD for short. You may have heard of THC—it is the main chemical responsible for the psychoactive effects of cannabis. Cannabis containing more THC produces a stronger effect, a higher high. CBD is less well known because it does not produce the same rewarding effects as THC and some researchers think this is key to its potential medicinal applications.

Recent research conducted by Dr. Yasmin Hurd from Mount Sinai Healthcare System in New York City suggests that CBD could be instrumental in combatting drug addiction and in particular heroin addiction. Remember the scene in Trainspotting when Euan McGregor’s heroin addict character goes cold turkey in his childhood bedroom? The disturbingly warped images in that scene tell us something of how brain function can be impacted by addiction. Like cannabinoids, heroin and other opioid drugs also operate by imitating natural chemicals in the body—they mimic endorphins, chemicals responsible for feelings of intense pleasure. Heroin addicts are typically prescribed another opioid, methadone, which replicates some of the pleasurable effects of heroin. But methadone is addictive too, meaning one addiction is often replaced by another. The craving for the next hit and the intense anxiety about when it will come remain, making relapse common.

This is where CBD comes in. As argued by Dr. Hurd in a recent Trends in Neuroscience article, the ability of CBD to modulate anxiety is a “highly reproducible and indisputable finding” of animal and human-based studies. When heroin addicts in New York were administered CBD in a pilot trial, they experienced less cravings for heroin when faced with cues associated with the drug. This reduction in cravings and anxiety continued for a week after CBD was last administered. Crucially, because CBD does not induce any pleasure, it is not addictive.

Exactly how CBD achieves the effect of reducing anxiety appears to be related to how the natural compound endocannabinoids operate in the body. As well as acting as neurotransmitters, endocannabinoids also act as neuromodulators—they put the brakes on reactions. In one study, images of human brains in patients undergoing a negative experience looked different depending on whether CBD had been administered. The difference was in the amygdala, a lobe of the brain heavily involved in anxiety, fear and stress. Activation of the amygdala was reduced in patients who had taken CBD compared to those who hadn’t.

More clinical trials are urgently needed to replicate results but a positive step towards the treatment of heroin addiction has been achieved and it originates in a compound derived from another illegal drug (in the UK, cannabis is still a Class C drug). Some may argue that such a remarkable, paradoxical potential solution has only been explored thanks to the sustained, committed public belief in the medicinal effects of cannabis.

Image credit: M. Martin Vicente