While we discuss cannabis oil, we should remember that the government is blocking life-saving research into MDMA and magic mushrooms

Cannabis – and specifically the use of cannabis oil – has been in the news a lot lately, spearheaded by the mother of epileptic Billy Caldwell, who had been successfully treated with oil containing THC but then denied it when his mother had the medical substance from Canada confiscated at the UK border. Science can be quite sniffy about single case studies like Billy Caldwell’s, viewing this as “low status” evidence compared to other methods of research. But many critical discoveries have been made by experiments based on one person. The way Charlotte Caldwell presented the effectiveness of cannabis oil in reducing her son Billy’s seizures has produced changes in government thinking that science has failed to do, despite decades of work challenging the legal status of cannabis.

The Caldwell case has ignited not one, but two debates about cannabis: whether it should it be legalised, and what the medicinal benefits of the drug – or indeed all illegal drugs that still might be medically useful – actually are. Both debates should be informed by evidence. Evidence should help cut through all the noise that often accompanies those who have strongly held views on either side of the argument.

The home secretary has said he will rely on evidence in his review of the potential health benefits of cannabis, but has ruled out any such parallel review of the recreational use of cannabis. Whenever someone refuses to use evidence for a review, we have to question why.

Evidence-informed policy is a relatively new idea in politics, and explains why we might be in the mess we are now in relation to cannabis. The 1971 Misuse of Drugs Act, which restricted the use of cannabis in the first place, was legislation constructed in an evidence-free way. Since then, all governments have inherited a regulatory system that has been informed more by politics than facts.

For example, the link between cannabis and mental health is often cited by the home secretary as justification for keeping it criminalised. But research exploring this relationship is messy, as among other problems, people tend to use more than one drug so we can’t absolutely establish causal relationships. It’s also never been clear whether people with conditions like schizophrenia are more likely to use cannabis because it controls their symptoms, or whether cannabis causes schizophrenic symptoms in the first place: as is often said in science, correlation doesn’t always mean causation.

We also have no agreed standard measure of cannabis strength or even classifying the type of cannabis use – sometimes we just ask in a binary way whether someone uses cannabis or not. This might seem petty but this detail matters, as studies can’t be reliably compared and individual studies may have under or overestimated the problem.

So the risks to health are based on messy evidence but on the other side – health benefits – there are problems too. Both personal stories and science support the case for cannabis providing therapeutic benefit, but such benefits are limited to specific problems. So although Billy Caldwell has responded well to cannabis oil, this doesn’t mean cannabis is effective for all types of epilepsy.

Likewise, there are all sorts of claims made in relation to cannabis, including its ability to shrink tumours or be effective in treating obesity. So there is a real danger that people are given false hope or worse are deliberately misled. Here we could learn a lesson from history in the way that the tobacco and alcohol industry have sought to distort policy and evidence in their favour in the past (who could forget the time when doctors used to prescribe smoking as a way to decrease anxiety?) The commercial cannabis industry is likely to do the same with claims of health benefits and interfering with policy to further their own interests.

Our understanding of how drugs like cannabis work is limited. But it is not just cannabis that offers potential health benefits. There is promise in others, such as MDMA – the chemical name for what is more widely known as ecstasy – or psilocybin, the active ingredient in magic mushrooms.

Knowledge has partly been held back by propaganda, much of which has tainted these drugs as addictive, which they are not. But this has restricted research funding and interest. That’s slowly changing, with renewed interest in the potential value of these drugs – some promising things have been achieved with both, particularly in the treatment of depression and for people who have experienced psychological trauma.

Trials so far have been on a small scale and will need to be replicated, ideally on a larger scale, but the results are encouraging and come at a time when there are few new drugs coming to market to treat mental health problems. Antidepressants don’t work for everyone, and then many are left with nowhere to turn. The government makes it intentionally difficult for researchers to work with drugs like MDMA and psilocybin despite the fact that these drugs could be potential life-savers; that’s as important as cannabis oil is for epilepsy, even if mental problems appear less straightforward or media-friendly than physical ones.

If evidence is to inform any change in policy on cannabis and other drugs, some voices will have more influence than others – we can’t allow politicians to speak over or even stand on an equal footing with scientists. The home secretary’s announcement that while he is willing to review the evidence on the medicinal properties of cannabis he won’t do this for recreational use is a classic example of why politicians’ voices need to be moderated. There are very good reasons for privileging scientific consensus here – for cannabis legalisation and beyond.

Ian Hamilton is a lecturer in mental health in the Department of Health Sciences at York University