Nicky Haynes has almost forgotten the pain she used to be in. She has been taking Sativex, a drug derived from cannabis, for about five years and says it has hugely improved her life. She was diagnosed with multiple sclerosis in 1994, and her condition progressively worsened to the point where she couldn’t sleep, where reaching for a glass of water would trigger a spasm and she was in constant pain. The first time she took Sativex was during a long car journey, fearful that she wouldn’t be able to walk when she got out. But she was able to walk better than she had for a long time.

After repeated, and failed, attempts to get the drug prescribed free on the NHS, Haynes has spent more than £20,000 on it. She can barely afford it – the family has to find about £500 a month from her benefits and workplace pension from a job she had to give up because of her illness. “My partner is my carer,” she says. “We go without to [pay for] it, and so far, it is worth it.” But her children are getting older and it is becoming harder to choose to spend the money on herself, even though it has, she says, given her freedom to do simple things such as get dressed, eat and drink, and use her wheelchair – as well as freedom from relentless pain. She has tried all other medications available: one made her hallucinate and talk nonsense to her children, she says.

The Liberal Democrats have pledged to legalise cannabis; at last year’s SNP conference, delegates backed the decriminalisation of the drug for medical use. But governments have always been fearful, worried that it might lead to legalisation for recreational use. Possession of the class-B drug can carry a five-year sentence, while suppliers face a 14-year sentence.

Sativex is an oral spray derived from cannabis that can help control pain. Photograph: Sativex

It is legal to prescribe Sativex, an oral spray, for MS sufferers but the National Institute for Health and Care Excellence (Nice) guidelines state it is not cost-effective and so it is extremely difficult to get on the NHS in England, Scotland and Northern Ireland, although the NHS in Wales does fund it. So people have to pay privately, or experiment with cannabis bought illegally – Haynes tried this but she wouldn’t want to do it again. “Can you imagine being caught with cannabis? My daughter is a teenager: they’ve had the drugs talk at school and she’d be mortified if her mum was caught.” And anyway, she says, getting the dosage right is hard because there’s no way to know what she would be buying.

Michael Barnes, consultant neurologist and honorary professor of neurological rehabilitation at the University of Newcastle-upon-Tyne, estimates that about half of his patients are using street-bought cannabis, “obviously with some benefit, otherwise they wouldn’t be risking taking an illegal product”.

Last year, Barnes, who also advises Clear, a group that campaigns for the legalisation of cannabis, published a report into medical use for the all-party parliamentary group on drug-policy reform. “The key indications that I think are unarguable – although there is more work that needs to be done – are for [treating] spasticity, a big problem after stroke, brain injury or multiple sclerosis. There is good evidence for helping pain in all sorts of contexts. There is surprisingly good evidence for anxiety, and nausea and vomiting, particularly in the context of chemotherapy.” There is also evidence that it could be useful for appetite stimulation, and treating symptoms of fibromyalgia and epilepsy. This year, in what’s believed to be a first for the NHS, a GP in Northern Ireland prescribed cannabis oil to an 11-year-old boy with epilepsy after the child’s mother took him to be treated by doctors in California, where medical marijuana is legal. The oil contains the compound CBD (but not THC, the psychoactive ingredient in the drug) and is also legal in the UK, reclassified as a medicine by the Medicines & Healthcare products Regulatory Agency late last year.

Like any drug, medicinal cannabis can have side-effects. “We found that the evidence, although it is quite conflicting, was that it can cause a psychotic episode in those who have a family history of schizophrenia,” says Barnes. Sensible doctors wouldn’t prescribe it to people at risk, he says. “We went through the other potential problems – such as, does it cause cancer if you smoke it? The answer is nobody knows, but we’re not recommending you smoke it anyway. Does it cause memory problems? Yes, it probably does, in heavy recreational users in the short term.”

David Casarett, author of Stoned: a Doctor’s Case for Medical Marijuana, started researching cannabis after a patient asked if it would help her. As a doctor in palliative care in the US, he had to confess he didn’t know much about it. When he started looking into it, the positives surprised him, he says. However, he adds: “There aren’t as many benefits as the most vocal proponents would have us believe. It’s pretty good for pain that’s due to nerve damage, muscle spasms, nausea and, potentially, insomnia. It’s not a wonder drug. It does not cure cancer [as some claim]. It does not, as far as we know, work for other conditions. It’s often described as a cure for PTSD, but there is no good clinical trial.”

Likewise, “there are not as many risks as the opponents of medical marijuana want us to believe,” Casarett says. He practises in a state where cannabis is illegal, so can’t recommend it to his patients. “If they’re using it, I caution them about using it safely – they should know there is a risk of addiction; they should know that it’s not safe to use and drive or operate heavy machinery. If they’re using it for a condition for which there is no evidence, I would tell them that.” Medical marijuana could be grown under controlled conditions, he says, and tested so you know what you are getting. “There’s also the not-insignificant benefit that you’re not supporting illegal drug trafficking.”

Barnes says he is “not suggesting we legalise cannabis generally [but] for medical purposes. I cannot, in all honesty, think of a sensible, evidence-based reason why it should not be.”