When Australian doctors prescribe medical cannabis products containing as much as 88% THC, you might assume there’s solid science behind those decisions. A new investigation says otherwise — and the findings have major implications for patients, regulators, and the global cannabis industry.
A rapid review published in Drug and Alcohol Review in 2026 by researchers at Monash University’s Addiction Research Centre has uncovered an uncomfortable truth: when it comes to the strongest THC-dominant medical cannabis products on the market today, there is essentially no clinical evidence of either safety or effectiveness. The full open-access study is available through Wiley Online Library, and its conclusions are already feeding into Australia’s ongoing regulatory overhaul.
The team, led by Myfanwy Graham, screened nearly 10,000 scientific records and ended up with just 15 studies that actually examined products comparable to Australia’s so-called “Category 5” cannabis — the THC-dominant class that has exploded in popularity over the past three years. The highest THC concentration tested in any of those 15 studies was 22%. Meanwhile, products containing up to 88% THC are being prescribed every day.
Key Takeaways
The review identified only 6 randomised controlled trials and 9 observational studies on Category 5 cannabis, with THC content ranging from 16% to 22% — far below the 88% maximum available in pharmacies.
There is no published clinical evidence on the safety or efficacy of products with THC concentrations above 22%, despite massive growth in prescriptions.
Reported side effects were mostly mild to moderate and included dizziness, cough, drug high, nausea, anxiety, and difficulty stopping use.
Not a single included study used a validated screening tool to measure cannabis use disorder, a significant gap given that an estimated 25% of medicinal cannabis users may develop dependence.
In Australia, 75% of Category 5 approvals are for chronic pain and anxiety, yet zero RCTs on anxiety met the inclusion criteria.
A Market That Grew Faster Than the Science
To understand why this matters, you need to picture what’s happened in Australia since 2021. After medicinal cannabis was legalised in 2016, the market was initially dominated by gentle oral oils and tinctures. Then something changed. Between 2022 and late 2024, prescription approvals for high-THC inhaled flower products surged past 250,000, overtaking oral liquids almost overnight. Half of all current medical cannabis approvals are now for Category 5 products.
The Therapeutic Goods Administration defines Category 5 as cannabis that is over 98% THC (or other non-CBD cannabinoids), with less than 2% cannabidiol. The category captures everything from modest 13% flower up to concentrated extracts touching 88% THC — concentrations that simply do not occur naturally in the plant and are produced through industrial extraction.
That regulatory bracket is enormous. And as Graham’s team points out, a 0.5 mg THC dose from a tiny inhaler puff is wildly different from rolling a joint with 88% extract. Treating them as one product class for evidence purposes is, in their words, increasingly indefensible.
What the Few Available Studies Actually Found
The handful of rigorous trials the reviewers could find tell an inconsistent story. In a small Canadian trial involving 16 patients with advanced chronic obstructive pulmonary disease, a single inhaled dose of vaporised cannabis at 18.2% THC produced no meaningful improvement in breathlessness or exercise endurance. Six participants coughed after inhaling, and five of those said their breathing actually got worse.
A more promising Israeli trial used a precision metered-dose inhaler delivering tiny 0.5 mg and 1 mg doses of 22% THC flower to people with chronic nerve pain. The 1 mg dose produced a clinically meaningful pain reduction of nearly 40%. But this was a single-dose study — what happens with weeks or months of repeated use was not examined. The result echoes a growing body of work suggesting that low-dose cannabinoids may rival conventional analgesics for certain pain types, as we covered in our recent analysis of how cannabis stacks up against ibuprofen for chronic pain.
A trial in patients with ulcerative colitis using smoked cannabis cigarettes (16% THC) found symptom improvement but no change in objective markers of inflammation. Notably, five of the seventeen people in the cannabis group reported “difficulty stopping use,” and seven later declined follow-up because they wanted to quit cannabis entirely.
In a fibromyalgia pain experiment, the strongest THC product (22%) produced a significant “drug high” in 80% of participants — most of whom said they disliked the feeling — but had inconsistent effects on actual pain measures. That mixed result aligns with newer neuroscience work exploring how THC alters the brain’s pain-filtering circuits in fibromyalgia patients, suggesting the drug may change pain perception rather than reduce pain signals directly.
The Side Effects Doctors Are Seeing
When you consolidate the adverse events across all 15 studies, a familiar pattern emerges. The most common complaints were dizziness, cough (especially with inhaled forms), dry mouth, nausea, anxiety, sleepiness, headache, and what researchers diplomatically called “drug high.” Most were rated mild to moderate and resolved when doses were reduced.
But there were warning signs. In a pilot study of cancer patients using THC capsules, three of four participants on a 10 mg dose dropped out due to side effects including hallucinations, anxiety, and disorientation. Even at half the dose, nearly a quarter of patients withdrew. Serious adverse events also appeared in Italian pharmacovigilance data, including arrhythmias linked to oral 22% THC products.
The review also references Australian case reports of psychosis, severe anxiety, and cannabis hyperemesis syndrome connected to Category 5 prescriptions — incidents that helped trigger the review in the first place.
The Cannabis Use Disorder Blind Spot
Perhaps the most striking finding is what’s missing. Not one of the 15 studies used a validated screening tool to detect cannabis use disorder. This matters enormously, because a separate 2024 systematic review estimated that roughly one in four people using cannabis for therapeutic purposes meets the criteria for the disorder. Younger age, male sex, inhaled use, and mental health conditions all raise the risk further.
Without proper screening, the clinical trials underpinning these products are essentially blind to one of the most predictable downsides of long-term THC use. Graham’s team flags this as an urgent priority for future research.
What This Means for Patients and Regulators
The authors stop short of demanding a ban, instead arguing for a more nuanced approach. They acknowledge that abruptly removing Category 5 access could harm patients who rely on it — particularly those in palliative care — and could trigger withdrawal in long-term users. Time to titrate down, transition to other categories, or shift to lower-potency alternatives matters.
But they are direct about one conclusion: the widespread prescribing of ultra-high-THC products in Australia is not supported by the evidence base. Clinical guidelines from bodies like the Royal Australian and New Zealand College of Psychiatrists have started to incorporate cautions about these products, but integration across health professional resources remains patchy. Public education for patients, the authors argue, is also overdue.
For patients considering high-THC products, the practical takeaway is straightforward. If you’re prescribed Category 5 cannabis, work closely with a clinician who monitors your symptoms, side effects, mental health, and overall function. Start low and go slow — the studies that did show benefit used surprisingly small THC doses. And be honest with yourself and your prescriber about any signs of dependence.
The deeper lesson is one the cannabis industry will probably have to grapple with worldwide: regulatory frameworks that have allowed product potency to climb steeply have, so far, outrun the science needed to determine whether those products help or harm. As Graham and her colleagues put it, this is a notable gap. Closing it will require high-quality, longer-duration trials that incorporate real safety surveillance — including for dependence — and that test the products patients are actually being prescribed, not just the ones researchers find convenient to study.
FAQ
It’s an Australian regulatory category for medical cannabis products that are predominantly THC, with cannabidiol making up less than 2% of total cannabinoids. The category covers everything from around 13% THC flower to extracts containing up to 88% THC, all of which can require a doctor’s prescription through Australia’s Special Access Scheme.
The available research suggests common side effects are usually mild to moderate, including dizziness, cough, nausea, and anxiety. However, this review found no clinical studies at all for products above 22% THC, even though products of 60% to 88% are widely prescribed. Australian case reports have linked these stronger products to psychosis, severe anxiety, and cannabis hyperemesis syndrome, so caution is warranted.
The evidence is mixed and limited. One trial showed meaningful pain reduction with a tiny 1 mg THC dose for nerve pain, but most studies were single-dose or short-duration and did not assess long-term clinical outcomes. For anxiety — the second most common reason these products are prescribed in Australia — no randomised controlled trials met the inclusion criteria, meaning there is essentially no high-quality evidence supporting this use.
Yes, this is a real and under-studied risk. International research estimates that around 25% of people using cannabis therapeutically may develop cannabis use disorder. None of the 15 studies in this review used validated screening tools to detect dependence, so the true rates among Australian patients are unknown — a gap the authors describe as urgent.
Disclaimer: This article summarises peer-reviewed scientific research for general informational purposes and is not medical advice. Medical cannabis is a prescription medicine with potential benefits and risks that depend on individual health conditions, other medications, and product composition. If you are considering medical cannabis or are concerned about your current use, consult a qualified healthcare professional. The views expressed are those of the article’s authors and journalists and do not necessarily reflect those of regulatory bodies or the original study’s funders.
