It has been linked to poor motivation, schizophrenia and paranoia.

But marijuana is actually a very effective drug and should be legalised to help patients who are in pain, a leading doctor argues.

Mike Barnes, honorary professor of neurological rehabilitation at Newcastle University, says there are an estimated three million cannabis users in the UK - one million of those who use it for medical reasons.

At the moment, all those individuals are forced to use it illegally.

Meanwhile, in the US, where the drug has been legalised, more than 1.2 million patients are prescribed medical marijuana.

Cannabis should be legalised for medical reasons as the drug can help treat conditions from chronic pain to cancer, argues Mike Barnes, honorary professor of neurological rehabilitation at Newcastle University

Professor Barnes, who has a long standing interest in the management of spasticity - a condition in which the muscles become stiff, causing pain or interfering with movement and speech - admits the drug carries risks and side effects.

However, it is effective at treating conditions from chronic pain to cancer, he says.

Writing for medical blogging site The Hippocratic Post, he says legalisation would allow robust studies to be carried out so the benefits and harms of the drug can be quantified.

It would also allow the product to be regulated, minimising risks associated with buying an illegal drug.

A long term advocate of the legalisation of cannabis for medical use, below, he explains his view...

CANNABIS IS AN EFFECTIVE DRUG

There is considerable evidence for cannabis being effective to treat a variety of conditions.

Certainly quite good evidence exists showing it is effective in helping patients with spasticity.

Indeed a cannabis product – Sativex – is prescribed legally to patients with multiple sclerosis if their spasticity has become resistant to other drugs.

It has also been shown to reduce the number of times they need to empty their bladder.

However, cannabis has also shown to be useful in chronic pain, particularly non-cancer pain and neuropathic pain, which is caused by damage to the brain or nervous system.

Good evidence shows cannabis is effective in helping patients with spasticity, a condition in which the muscles become stiff, causing pain or interfering with movement and speech, Professor Barnes says (file photo)

There are a whole variety of other conditions in which evidence shows it can be useful, for example managing the nausea that comes with chemotherapy and improving sleep.

It can be used as an appetite stimulant for patients in debilitating conditions, such as those with HIV.

There is also some evidence of efficacy in fibromyalgia, a condition characterised by muscle pain and stiffness, and rheumatoid arthritis, where the joints become inflamed and painful.

It is also useful to treat inflammatory bowel disease, migraine, glaucoma - where pressure builds up in the eye, affecting sight - and some forms of dystonia - the medical name for muscle spasms and contractions.

There is particularly good evidence of efficacy in a type of childhood epilepsy, Dravet’s syndrome, where sufferers have become resistant to drugs.

Cannabis also has anti-anxiety and anti-psychotic properties

A NATURAL ANTI-ANXIETY MEDICINE

Marijuana is made from more compounds called cannabinoids (there are over 70 in natural cannabis), some of which are known to have a whole variety of interesting properties.

One such cannabinoid is cannabidiol (CBD), a component of natural cannabis which has anti-anxiety and anti-psychotic properties.

It can also help reduce ‘fear’ memory, such as in post-traumatic stress disorder and in some phobias.

It may have anti-inflammatory and brain-protecting properties.

Overall there is good evidence of the efficacy of cannabis and indeed it has been used as a medicine for over 4,000 years.

A DRUG WITH FEW SIDE EFFECTS

Fortunately, cannabis has few side effects and the ones it does trigger are mild and well tolerated.

They include dizziness, dry mouth, fatigue, sometimes nausea and some balance difficulties.

Of course, in the cannabis that has relatively high THC (tetrahydrocannabinol – the psycho-active component) there is the ‘high’ which is sought by the recreational users.

Around 10 per cent of people have troublesome but nevertheless tolerable side effects.

Less than 1 per cent of individuals drop out of cannabis studies, which compares to about a 33 per cent drop out rate in opioid studies.

Opioids are are a group of pain-relieving drugs which include morphine, codeine and heroin.

LEGALISING CANNABIS LEADS TO BETTER QUALITY DRUGS

Making cannabis legal will of course take it out of the illegal drug market.

At the moment people who wish to buy cannabis have to do so through illegal drug pushers and much of the cannabis purchased is of unknown and sometimes dubious quality.

If it is prescribed, then the cannabis can be made of predictable and high quality.

The Sativex compound, for example, is of medicine standard and consists of 50 per cent THC and 50 per cent CBD.

There are many different types of cannabis with varying ratios of the key cannabinoids.

The drug can help managing the nausea that comes with chemotherapy, improve sleep and can be used as an appetite stimulant for patients in debilitating conditions, such as those with HIV, he added (file photo)

Some people may benefit from a relatively high THC component and some from a relatively high CBD component.

Growers can produce products that can be ‘tuned’ to the various conditions for which it is helpful.

Making medical marijuana legal would iron out anomalies in the law.

Sativex is a legal product, while dronabinol, a synthetic THC used to treat nausea during chemotherapy, is also legally prescribed.

Cannabis is an effective drug in treating chronic pain, cancer, migraines, inflammatory bowel disease, childhood epilepsy as well as improving sleep and stimulating the appetite Professor Mike Barnes, expert in neurological rehabilitation at Newcastle University

And CBD in isolation is not a banned product.

Thus the key components of cannabis are in various forms legal yet the use of herbal cannabis (which has less side effects than synthetic products) remains illegal.

Frankly, this is rather a daft situation as 11 European countries, several other countries around the world and 24 US states have now made cannabis legally prescribable.

They have determined that the benefits outweigh the risks.

THE RISK OF SCHIZOPHRENIA

Most objections focus on the long term side effects.

There is some evidence that heavy recreational users of cannabis are at a higher risk of schizophrenia or other psychotic episodes.

There is certainly a relationship between higher doses of cannabis and the mental illness, with schizophrenia mainly triggered in heavy users or those with a family history of psychosis.

Obviously this is of some concern but the risk could be mitigated if doctors simply made them clear to patients.

The drug does carry side effects: Heavy recreational users of cannabis are at a higher risk of schizophrenia or other psychotic episode

Other authors have noted memory and thinking problems in heavy recreational users, but these problems seem to focus on a reduction of verbal memory in the longer term.

Once again, this is of some concern, but a counter-argument is that legalising cannabis would make proper long term studies possible so these risks can be quantified and action taken to reduce them.

Other studies claim cannabis reduces the volume of the brain, particularly around the hippocampus - thought to be the centre of emotion, memory.

It also controls the autonomic nervous system, which controls unconscious actions such as breathing, our heartbeat and digestive processes.

However, the significance of these findings is not certain.

IS THERE A RISK OF DEPENDENCY?

The studies show that around 9 per cent of cannabis users become dependent on the product, which compares to around 32 per cent of tobacco users and 15 per cent of alcohol users.

A useful article in the Lancet in 2010 convincingly demonstrated that cannabis was overall less harmful than tobacco and alcohol.

Studies in some of the US states, where growing one’s own cannabis is legal, have shown that there is a ‘diversion’ rate.

In other words, some of the cannabis grown legally for medical reasons is diverted into the recreational market.

Indeed, in Colorado it has been shown that there is four times more production of cannabis than is required for medical use within the state.

This is a risk but it is a risk that is much reduced if cannabis is produced by licenced growers who can control the quality and ensure, with licenced pharmacists, a secure supply chain.

Legalising the drug would allow for robust studies to be carried out on its effectiveness in treating medical problems, and its side effects, Professor Barnes said. Pictured, a multiple sclerosis patient

PRESCRIBING CANNABIS COULD HELP PEOPLE IN PAIN

Overall there are good reasons to prescribe cannabis.

Certainly there are some concerns about long term usage.

However, legalisation of cannabis for medical conditions would help around a million people in the UK every year who are in chronic pain or troubled by neurological and other conditions.

It would also allow properly controlled studies to be undertaken to determine with accuracy the effectiveness and side effects of the product.

At the moment there is a campaign to make medical usage legal – see endourpain.org – and I would encourage readers to sign that petition.