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Thousands of people to be given cannabis as a painkiller every day in major new trial 

The latest review, by experts from Oregon Health and Science University, was published in Annals of Internal Medicine.

It looked at evidence from 18 placebo-controlled studies and seven long-term papers from around the world.

A total of 14,835 people were involved in the trials and took the products for up to a year.

Patients were asked to assess their pain before, during and several weeks after taking cannabis-based products.

Results showed some synthetic products with high THC content had a 'moderate effect' on reducing pain, causing pain to fall one to two points on the scale.

But they had a 'large effect' on causing dizziness, a 'moderate effect' on tiredness and 'potential effect' on nausea.

Using the cannabis plant itself did not show a statistically significant benefit and the results were marred by methodological issues, including not recording dose levels or side effects.

The authors said: 'Evidence for whole-plant products, CBD, and other cannabinoids was limited by serious imprecision and lack of ability to assess consistency and study methodological limitations. 

'These studies were not designed to evaluate harm outcomes and often excluded patients who were at higher risk for harms, potentially underestimating the harms of cannabinoid treatment.'

Psychosis and cannabis addiction were also not considered by the 'whole plant' studies, they said, which is one of the main concerns about using the drug.

Lead author Professor Marian McDonagh, an epidemiologist at the university, said the review found a surprising lack of evidence for using cannabis to treat pain.

She said: 'In general, the limited amount of evidence surprised all of us.

'With so much buzz around cannabis-related products, and the easy availability of recreational and medical marijuana in many states, consumers and patients might assume there would be more evidence about the benefits and side effects. 

'Unfortunately, there is very little scientifically valid research into most these products.

'We saw only a small group of observational cohort studies on cannabis products that would be easily available in states that allow it, and these were not designed to answer the important questions on treating chronic pain.' 

Writing in a commentary in the same journal, University of Michigan anaesthiologists Professor Daniel Clauw and Dr Kevin Boehnke said the study was unable to prove the benefits of cannabis due to study limitations.

They said: 'These limitations are well documented in the cannabinoid and chronic pain literature and are due in part to "War on Drugs" policies that have overwhelmingly favored studying cannabis-related harms over therapeutic effects. 

'Unfortunately, this means this well-conducted review found limited generalisable evidence to inform long-term use of available cannabis products for chronic pain.'

But the researchers called on doctors to be pragmatic when dealing with patients who want to try using cannabis for pain relief. 

They said: 'Conventional analgesic medications are effective only in a subset of persons, so it is no wonder that many patients are drawn to widely available cannabis products. 

'Clinicians can compassionately witness, record, and offer guidance to help patients with chronic pain use cannabis wisely.'