There is a large gap between the public interest in medical marijuana and the scientific evidence. In recent weeks, we have learnt that the Government will make it available for three specific medical conditions and that - under close medical supervision - its impact will be monitored.
The term medical marijuana refers to using the whole unprocessed marijuana plant or its basic extracts to treat a disease or symptom. The US Food and Drug Administration (FDA) has not recognised or approved the marijuana plant as medicine. The chemicals in marijuana, called cannabinoids, have been studied and this has led to two FDA-approved medications that contain these chemicals being produced in tablet form. The plant contains more than 60 pharmacologically active cannabinoids.
One of these is tetrahydrocannabinol (THC), marijuana's main mind-altering ingredient. Another, cannabidiol (CBD) is of particular interest in treating certain conditions such as childhood epilepsy. THC and CBD are the two main cannabinoids that are of medical interest.
THC increases appetite and reduces nausea. The FDA-approved THC-based medications are used for these purposes. THC may also decrease pain, inflammation (swelling and redness), and muscle control problems.
CBD is a cannabinoid that does not affect the mind or behaviour. It may be useful in reducing pain and inflammation, controlling epileptic seizures, and possibly even treating mental illness and addictions.
Two FDA-approved drugs, dronabinol and nabilone, contain THC. They treat nausea caused by chemotherapy and increase appetite in patients with extreme weight loss caused by AIDS and have been approved in the US since 1985.
In Ireland, a register of the use of marijuana will be established as part of this process of monitoring it. The three medical conditions in which it will be allowed are for chemotherapy associated nausea and vomiting that does not respond to other medications, for the spasticity associated with multiple sclerosis and for severe epilepsy.
This comes on foot of public demand, and also from a government-appointed review group headed by Prof Tony O'Brien, a Cork-based oncologist. "Access to medicinal cannabis is ultimately a societal and policy decision which has to balance the lack of scientific evidence against patient-led demand," the report by the group said. This is the nub of the issue - the evidence is limited, and according to the report from the group "at times conflicting".
The barrier to quality studies is gradually being eroded but there are still few controlled studies, and belief in its benefits is often based on surveys of people obtaining illicit cannabis to treat their condition or on animal studies. So what is the evidence?
Intractable childhood epilepsy has been one of the most publicly-supported conditions that campaigners have used in calling for its availability. In the UK, epidiolex, a CBD-based drug to treat certain forms of childhood epilepsy, has been synthesised and has been tested in an open label study in the US. It showed a 36.5pc reduction in seizure frequency.
The effectiveness of medicinal cannabis in treating the symptoms of pain, nausea, vomiting and appetite in cancer patients is recognised by the Clinical Oncology Society of Australia, but more research is still needed in these areas. There is currently insufficient clinical evidence to support the use of medicinal cannabis in preventing or treating cancer pain according to that organisation, although there is some evidence of benefit from one randomised controlled trial when used in combination with other agents.
The evidence supporting medicinal cannabis as a treatment for muscle spasticity and associated pain in multiple sclerosis is probably the strongest. Based on the available evidence, an oral extract consisting of a mixture of THC and CBD or dronabinol on its own has been recommended by the American Academy of Neurology.
A synthesis of all the published papers in this area was published in the prestigious Journal of the American Medical Association (JAMA) in 2015. It examined 79 trials involving 6,462 participants. Most trials showed improvement in symptoms associated with cannabinoids but these associations did not reach statistical significance in all trials. There was moderate-quality evidence to support the use of cannabinoids for the treatment of chronic pain and spasticity. There was low-quality evidence suggesting that cannabinoids were associated with improvements in nausea and vomiting due to chemotherapy, weight gain in HIV infection, sleep disorders or Tourette syndrome.
The calls for legalisation of medicinal cannabis have been shrill and perhaps based on misunderstanding. This is most certainly not about decriminalisation of cannabis, as perhaps some campaigners had hoped. It is more about applying the limited evidence that is available to very specific grave, clinical situations that can be closely monitored. For now the evidence is very limited and the studies do not yet meet the criteria normally required by authorities charged with licensing new medications, ie randomised, controlled, double-blind trials in large numbers of individuals. The judicious approach of our Government is the correct one.
Patricia Casey is Professor of Psychiatry at University College Dublin