Thousands of Australians are now using medicinal cannabis to treat conditions like chronic pain and anorexia.

Yesterday, the ABC revealed more than 3,100 medicinal cannabis scripts had been approved by the Therapeutic Goods Administration (TGA) since the Federal Government relaxed restrictions in March 2018.

According to experts, these people were just the tip of the iceberg. It has been estimated as many as 100,000 Australians self-medicate with cannabis they've acquired illegally.

Advocates of the drug say it offers a safe and effective solution to people with intractable medical conditions.

But critics, and some of Australia's leading medical experts, argue there's limited quality evidence to support the use of medicinal cannabis in most conditions.

So, what do we know about it? And why, in some cases, is it still so little?

The challenges of cannabis research

Research into the safety and efficacy of cannabis has traditionally been very difficult.

This is partly because of the illegal nature of cannabis, and partly because of the complexity of the cannabis plant itself.

Cannabis contains more than 400 bioactive molecules, about 100 of which are cannabinoids — a diverse group of natural chemicals that bind to the body's endocannabinoid receptors to produce various effects.

The two main cannabinoids that have been found to have therapeutic benefits are delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD).

THC is known for its psychoactive effects — it's what makes a person 'high', and is why people use marijuana recreationally. CBD, on the other hand, is not psychoactive, and is thought to moderate the 'high' caused by THC.

The ratios of THC and CBD (and other cannabinoids) determine choice of product for treatment of disease.

Different cannabis strains contain different ratios of THC to CBD, and it unclear whether they act individually or in conjunction with each other.

Because of the inherent variability of the cannabis plant, as well as the many ways it can be administered, the types of products, doses and research methods used in clinical studies have varied significantly.

This, according to the TGA, has made it difficult to "come to firm conclusions" about how best to use medicinal cannabis.

It's also why, with the exception of one product (used for muscle spasticity in multiple sclerosis), medicinal cannabis products are not available as registered prescription medicines.

What the evidence says

Despite suggestions medicinal cannabis may help with everything from chronic pain to reducing anxiety, scientific literature to date has painted a mixed, and largely inconclusive picture.

Amid growing public interest in the drug in recent years, the National Drug and Alcohol Research Centre (NDARC) conducted a systematic review of medicinal cannabis in 2017, which formed the basis of current TGA guidelines.

On the whole, the review found the evidence was "limited", and suggested cannabis only be used when registered medicines have been "tried and proven unsuccessful".

Jennifer Martin, director of the Australian Centre for Cannabinoid Clinical and Research Excellence, said it was better for patients to use drugs that had been registered by the TGA first.

"That's because current therapies that we've got have been assessed for safety, quality, and consistency," Professor Martin said.

Medicinal cannabis products involve plant-derived or synthetically-produced cannabinoids. ( ABC News: Jerry Rickard )

The TGA review found the strongest evidence for medicinal cannabis was in children and young adults with drug-resistant epilepsy, for which CBD products have been shown to reduce the frequency of seizures by 50 per cent or more in up to half of paediatric patients.

"This is probably where our best evidence is to date," Professor Martin said.

The review found there was "low to moderate" evidence for medicinal cannabis products helping with the pain symptoms of multiple sclerosis, although the evidence was inconsistent.

For chemotherapy-induced nausea and vomiting, the review found high-THC medicinal cannabis products were "as effective" as conventional drugs — but the review only compared marijuana with older, superseded drugs.

"A lot the studies were from the 1990s, before we had really good drugs for chemotherapy-induced nausea and vomiting," Professor Martin said.

"Now we have really good drugs, there's not really a need for cannabis products in this space."

For chronic pain, there was some evidence medicinal cannabis could assist with neuropathic (nerve) pain — but for most patients the effect was modest.

However, a large-scale 2017 review from the National Academy of Sciences, Engineering, and Medicine in the US found "substantial evidence that cannabis is an effective treatment for chronic pain in adults".

"But when the Australian team updated the information, they found we would have to treat 24 patients with this particular cannabinoid for one of them to get a reduction in their pain symptoms," Professor Martin said.

"Certainly some people that have taken cannabinoids do say they have had a lot of benefit, but we have also seen older patients who have had side effects."

Professor Martin said although medicinal cannabis was likely to be less harmful than opioid medication, the solution to chronic pain wasn't as simple as moving someone off one pain medication and on to another.

"It's an important question that needs further research … but I think my colleagues in the pain medicine and addiction medicine community would tell you it's not that straightforward," she said.

When it comes to palliative care, the NDARC review found there was little evidence that medicinal cannabis benefited advanced cancer patients with chronic pain.

However, Michael Farrell, who is director of NDARC and helped author the 2017 review, said the main concerns about medicinal cannabis were centred around long-term usage — which are "not an issue" when it comes to end of life.

"I can't see why we should worry too much about complex evidence if it's for short-term management," Professor Farrell said.

Taking a cautious approach

Iain McGregor from the Lambert Initiative For Cannabinoid Therapeutics at Sydney University said although much more research into medicinal cannabis was needed, it was important not to conflate "poor evidence" with an "absence of evidence".

Iain McGregor, academic director of Lambert Initiative For Cannabinoid Therapeutics at Sydney University. ( ABC News: Jerry Rickard )

"In many cases the TGA guidelines state that good quality evidence has not been done in a particular area," Professor McGregor said.

"That's a very different outcome to good quality studies have been done and cannabis was ineffective.

"Often it’s just the case that the studies haven’t yet been done."

Professor McGregor said he thought Australia was "erring too far on the side of caution" in its approach to medicinal cannabis.

"I think other similar OECD countries have decided to give cannabis the benefit of the doubt, particularly in patients who are desperate," he said.

But Professor Martin said Australia was right to take a slow and steady approach to developing knowledge about the efficacy and safety of medicinal cannabis.

"Patients have a short-term solution — they can get access through the TGA system," she said.

"But long term, we still have to collect rigorous data and make sure our patients are safe before we roll out open access to a product that we don't really know how to use properly."

Professor Farrell agreed and said it was "important people have realistic expectations of what medicinal cannabis might do, other than thinking it's some sort of magic bullet".