There are few substances that excite discussion quite as potently as cannabis. Explosive claims about its curative power circulate wildly online, often cited by proponents of medical marijuana. A quick Google search for ‘cannabis cures’ yields anecdotes of miraculous efficacy for every illness imaginable, and particularly for cancer. But, despite the gushing testimonials, this persistent narrative of cannabis as a universal panacea is distinctly at odds with the evidence.
In a review earlier this year of over 10,000 studies, researchers found reliable evidence for three distinct applications of cannabis. First, there is strong evidence that tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, can reduce nausea and vomiting associated with cancer treatments. These anti-emetic properties have been exploited for decades in the clinical management of cancer symptoms.
However, there is a caveat to this. THC is not uniformly well-tolerated and in many instances can exacerbate rather than placate vomiting. As safer and more efficient medications exist, THC-derived clinical compounds tend only to be used for this purpose when other interventions have failed.
The review also found good evidence to support the use of medicinal cannabis in chronic pain, and in managing spasms associated with multiple sclerosis.
Such benefits are comparatively modest but certainly not negligible. But what of the other breathless claims of curative potency made by cannabis advocates? The same review found that, despite the hyperbole, the evidence for the efficacy of cannabis in other conditions was minimal and unconvincing.
In particular, there was no strong evidence that cannabis was useful for the treatment of epilepsy, appetite regulation for Aids patients, ADHD, Parkinson’s disease or irritable bowel syndrome. As Dr Sean Hennessy, one of the study’s authors, said: ‘Most of the therapeutic reasons people use medical marijuana aren’t substantiated beneficial effects of the plant.’
Yet, despite this, miracle stories abound. Autism is particular is often targeted by those selling bunkum elixirs, and cannabis is regularly touted as a treatment despite the lack of evidence of benefit.
Nowhere are these claims so dangerous as when they’re made in relation to cancer. Cancer sufferers are frequently hawked dubious cure-alls, with cannabis perhaps the most common. There is absolutely no evidence that cannabis has any curative impact on cancers, as organisations like Cancer Research UK and the National Cancer Institute point out. This tends to go entirely unheeded. One recent meme tells a wondrous tale of a man who cured his cancer with cannabis oil. This has been shared widely by cannabis enthusiasts, seemingly unaware that the man named in fact succumbed to his cancer months before.
It is common for such miraculous anecdotes to be shared and hailed as proof with little concern about their veracity. Another frequent claim is that high-dose THC can kill cancer cells in a petri dish, which is true — but irrelevant. The astute reader will note that humans are not petri dishes, and killing cells can be achieved by many other agents, from heat to acid to bleach. Killing cells is easy — an effective anti-cancer agent has to discriminately target cancer cells while sparing healthy ones, and there is no evidence that cannabis can do anything of the sort. At best it’s cargo-cult science, a superficial veneer of credibility for views incongruent with the evidence.
This is damaging for several reasons. For one, it fosters a mistrust of the medical and scientific community. To preserve their faith, true believers dismiss the lack of evidence for their position as the machinations of Big Pharma. This casts researchers and medics as pantomime villains and makes them targets of abuse (as I can attest). Worse than that, the message is delivered so fervently that it persuades many patients to cease their conventional therapies. This has, and will, cost lives.
Cannabis advocates are less keen to entertain the notion that it might have any adverse effects. As a general rule of thumb, anything that has a biological impact is likely to have some potential ill effect, and cannabis is no exception. While relatively safe, regular users have an elevated chance of mental health disorders, including schizophrenia. For children and adolescents, these effects are much more pronounced, with negative implications for educational and social attainment. Contrary to popular misconception, addiction and problem usage are common, and more likely to manifest in heavy users and those who begin at a younger age.
None of this is to take away from the effectiveness of cannabis products for certain conditions, nor is it a discussion on legalisation. There may be good arguments for legalising cannabis — but exaggerating its potential use in medicine is not one of them.
If medicinal use is the true aim, then regulation of cannabis as a health product should not be controversial. That some activists lobby for ‘medicinal’ usage while denigrating regulation suggests their true motives are recreational. The related mantra that cannabis is natural and ergo harmless is textbook naturalistic fallacy. Cyanide and plutonium are also naturally occurring, but unrestrained ingestion would be unwise.
The reality is that most of the health claims made about cannabis are wrong-headed and devoid of evidence. They risk endangering the lives of the very patients they are supposed to help.
Dr David Robert Grimes is a physicist, cancer researcher and science writer and was the joint recipient of the 2014 Nature / Sense About Science Maddox Prize. On Twitter he is @drg1985