Taking cannabis for medical reasons should be made legal, says a cross-party group of UK politicians.
The All Party Parliamentary Group on Drug Policy Reform says there is clear evidence cannabis could have a therapeutic role for some conditions, including chronic pain and anxiety.
It says tens of thousands of people in the UK already break the law to use the drug for symptom relief.
But the Home Office says there are no plans to legalise the "harmful drug."
Plant cannabis contains more than 60 chemicals.
The All Party Parliamentary Group wants the Home Office to reclassify herbal cannabis under existing drug laws, from schedule one to schedule four.
This would put it in the same category as steroids and sedatives and mean doctors could prescribe cannabis to patients, and chemists could dispense it.
Patients might even be allowed to grow limited amounts of cannabis for their own consumption.
People with multiple sclerosis can legally take a cannabis-based medicine.
This licensed medicine, called Sativex, is a mouth spray and contains two chemical extracts (THC and CBD) derived from the cannabis plant.
Under current laws in England and Wales, cannabis is not recognised as having any therapeutic value and anyone using the drug, even for medical reasons, could be charged for possession.
The NHS warns that cannabis use carries a number of risks, such as impairing the ability to drive, as well as causing harm to lungs if smoked and harm to mental health, fertility or unborn babies.
The All Party Parliamentary Group on Drug Policy Reform took evidence from 623 patients, representatives of the medical professions and people with knowledge of how medical cannabis was regulated across the world.
Risks and benefits
Co-chair Baroness Molly Meacher said: "Cannabis works as a medicine for a number of medical conditions.
"The evidence has been strong enough to persuade a growing number of countries and US states to legalise access to medical cannabis.
"Against this background, the UK scheduling of cannabis as a substance that has no medical value is irrational."
The group commissioned a report by an expert in rehabilitation medicine, Prof Mike Barnes, which found good evidence that medical cannabis helps alleviate the symptoms of:
- chronic pain (including neuropathic pain)
- spasticity (often associated with Multiple Sclerosis)
- nausea and vomiting, particularly in the context of chemotherapy
And there was moderate evidence that it could help with:
- sleep disorders
- poor appetite
- post-traumatic stress disorder
- Parkinson's symptoms
But there was limited or no evidence that cannabis helps:
- dementia mood problems
- bladder function
- Tourette's syndrome
- Huntington's disease
- obsessive compulsive disorder
- gut disorders
- curb cancer growth
It found short-term side-effects of cannabis were generally mild and well tolerated, but that there was a link with schizophrenia in some long-term users.
"There is probably a link in those who start using cannabis at an early age and also if the individual has a genetic predisposition to psychosis. There should be caution with regard to prescription of cannabis for such individuals," says the report.
Also, there is a small dependency rate with cannabis at about 9%, "which needs to be taken seriously but compares to around 32% for tobacco use and 15% for alcohol use".
The evidence for cognitive impairment in long-term users is not clear but "it is wise to be cautious in prescribing cannabis to younger people, given the possible susceptibility of the developing brain", says the report.
Smoking cannabis in a joint rolled with tobacco can make asthma worse and probably increases the risk of lung cancer.
Prof Barnes said: "We analysed over 20,000 scientific and medical reports.
"The results are clear. Cannabis has a medical benefit for a wide range of conditions.
"I believe that with greater research, it has the potential to help with an even greater number of conditions.
"But this research is being stifled by the government's current classification of cannabis as having no medical benefit."
Cannabis is currently classified as a Class B drug, with possession carrying a maximum sentence of five years in jail or an unlimited fine.
Those supplying or producing cannabis face tougher penalties, with a maximum of 14 years in jail.
The drug comes in many different forms - hash is cannabis resin, while marijuana is the dried leaves and flowers of the plant.
A Home Office spokesman said: "There is a substantial body of scientific and medical evidence to show that cannabis is a harmful drug which can damage people's mental and physical health.
"It is important that medicines are thoroughly trialled to ensure they meet rigorous standards before being placed on the market.
"There is a clear regime in place, administered by the Medicines and Healthcare Products Regulatory Agency, to enable medicines, including those containing controlled drugs, to be developed."
About 24 US states, Canada, Israel and at least 11 European countries already allow access to cannabis for medical use.
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