There is very little evidence to suggest cannabis can help ease the symptoms of a raft of illnesses, scientists have said.

Medicinal cannabis has been legalised in 23 US states as a therapy to treat disease or alleviate symptoms, and a further seven states have legislation pending.

But despite the drug's legal status in many parts of the world, the debate rages as to whether it is effective.

A new study, in which scientists assessed the findings of 79 randomised trials including around 6,500 volunteers, found the evidence supporting the use of medical cannabis is weak.

Scientists reviewed the evidence from 79 randomised trials to assess claims that cannabinoids - compounds that are the active principles in marijuana - help ease various medical conditions

It found moderate-quality evidence to support the use of cannabinoids - chemical compounds that are the active principles in marijuana - for the treatment of chronic pain.

And weaker evidence still that the drug eased nausea and vomiting in chemotherapy patients, sleep disorders, and Tourette syndrome.

Dr Penny Whiting, from the University of Bristol, and her team evaluated the evidence for benefits and adverse events, related to medicinal cannabis use.

Researchers found most studies they examined suggested cannabinoid use was associated with an improvement in symptoms.

But these associations did not reach statistical significance in all the studies evaluated.

Moderate-quality findings suggest that cannabinoids may be beneficial for the treatment of chronic neuropathic or cancer pain, and muscle contractions due to multiple sclerosis.

But researchers discovered weak evidence that medical marijuana was associated with improvements in nausea and vomiting due to chemotherapy, weight gain in HIV, sleep disorders and Tourette syndrome.

And there was very-low quality evidence the drug eased anxiety.

In addition, Dr Whiting and her team found weak evidence to support the claim that medicinal cannabis has no effect on psychosis, and very little evidence in defence of the idea it had no effect on depression.

Meanwhile, the drug was linked with several adverse effects.

They found weak evidence that medicinal cannabis, which has been legalised in 23 US states, helps ease nausea and vomiting in chemotherapy patients, sleep disorders and Tourette syndrome (file picture)

Notably, cannabinoids were found to cause dizziness, dry mouth, nausea, fatigue, euphoria, vomiting, disorientation, drowsiness, confusion, a loss of balance and hallucination.

The study authors said: 'Further large, robust, randomised clinical trials are needed to confirm the effects of cannabinoids, particularly on weight gain in patients with HIV/AIDS, depression, sleep disorders, anxiety disorders, psychosis, glaucoma, and Tourette syndrome are required.

'Further studies evaluating cannabis itself are also required because there is very little evidence on the effects and adverse effects of cannabis.'

In an editorial linked to the study, Dr Deepak Cyril and Dr Mohini Ranganathan, of Yale University, said: 'Since medical marijuana is not a life-saving intervention, it may be prudent to wait before widely adopting its use until high-quality evidence is available to guide the development of a rational approval process,' they add.

'Perhaps it is time to place the horse back in front of the cart.'