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Published: 12 August 2026

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Abstract: Cannabis as a Gateway Drug for Opioid Use Disorder
Emerging evidence suggests that adolescent cannabis use may biologically and behaviourally increase vulnerability to later opioid use and the development of opioid use disorder (OUD). The paper argues that while shared risk factors—such as trauma, impulsivity, psychiatric symptoms, and genetic predisposition—contribute to both cannabis and opioid use, cannabis itself may introduce additional risk during critical neurodevelopmental windows.

Adolescence is marked by extensive neural pruning, synaptogenesis, and maturation of executive‑function circuits. The endocannabinoid system plays a central role in these processes, and exogenous cannabinoids can disrupt synaptic plasticity, dendritic architecture, glutamatergic signalling, CREB‑mediated gene expression, and NMDA‑dependent memory formation. As the paper notes, “cannabis can change adolescent gene expression and alter these key periods of neurodevelopment” . These alterations overlap with pathways implicated in opioid reinforcement and addiction.

Animal studies consistently show that adolescent THC or cannabinoid exposure enhances later heroin intake, increases conditioned place preference, induces morphine tolerance, and modifies striatal gene expression in ways that prime opioid sensitivity. Genetic variation (e.g., Penk, FosB expression) appears to moderate this priming effect, suggesting a subset of adolescents may be especially vulnerable.

Human population‑level studies mirror these concerns. Cannabis users are less likely to reduce opioid use over time, medical cannabis users show higher rates of prescription opioid misuse, and frequent cannabis use correlates with increased likelihood of meeting criteria for OUD. High‑potency cannabis may further amplify these risks. Additionally, cannabis‑related worsening of depression, suicidality, anxiety, and psychosis may indirectly increase opioid initiation among already at‑risk youth.

Overall, converging biological, developmental, genetic, and epidemiological evidence indicates that for some adolescents—particularly those with high‑risk genotypes or psychiatric vulnerability—cannabis use can meaningfully increase the likelihood of opioid initiation, escalation, and OUD.

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Also see  Dalgarno Research Report Cannabis and the Gateway Drug Theory: Correlation or Causation – Where does the Evidence Point?

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Published: 12 August 2026

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Rising rates of cannabis use worldwide have renewed scientific attention on a question long overshadowed by the tobacco debate: what does inhaling burnt cannabis actually do to the human airway? A newly published Canadian investigation, presented recently through the IASIC Speaker Series webinar "The Lung Health Implications of Cannabis Smoking," moderated by Dr Russell Kamer, MD, FACP, has added the most granular evidence yet that habitual cannabis smoking is not the respiratory non-event some casual users assume it to be.

The CANUCK Study: A Multi-Layered Investigation

The centrepiece of the new evidence is the Canadian Users of Cannabis Smoke (CANUCK) study, published in the European Respiratory Journal and led by a team including Dr Janice Leung, an Assistant Professor of Medicine at the University of British Columbia and a respirologist at St Paul's Hospital in Vancouver. The study is notable for combining clinical symptom reporting, lung-function testing, advanced imaging and molecular airway analysis in the same cohort, rather than relying on a single measure of harm.

Researchers found that cannabis-smoking participants reported measurably worse respiratory symptoms than people who had never smoked, and that those with the highest cumulative "joint-year" exposure showed reduced pre-bronchodilator FEV1/FVC ratios, lower mid-expiratory airflow, more radiographic emphysema and more ventilation abnormalities on imaging compared with never-smokers. At the cellular level, airway epithelial brushings from cannabis smokers revealed an altered immune signature — an increased type 2 immune response paired with a decreased type 17 response — alongside elevated expression of the MUC5AC mucus-producing gene, a change that correlated with worse clinical outcomes and imaging abnormalities in laboratory cell cultures.

Speaking about the findings, Dr Leung described the project as a deliberately comprehensive effort: "We decided to do a really detailed approach to cannabis smoking and its impact on the airway from clinical to imaging to functional outputs all the way down to the molecular airway cell level," noting that greater smoking severity tracked with worse respiratory symptom scores and more pronounced functional and imaging abnormalities.

Where the New Findings Sit Within the Broader Evidence Base

The CANUCK results reinforce, rather than overturn, a body of prior research that has consistently linked chronic cannabis smoking to chronic bronchitis-type symptoms — cough, sputum production and wheeze — even as evidence for classic airflow obstruction has remained more equivocal. A 2016 systematic review found that eight of nine studies measuring respiratory symptoms reported significantly higher complaint rates among cannabis smokers, with odds ratios as high as nearly three compared with non-smoking controls, while lung volumes (FVC) tended to increase rather than decrease with cannabis exposure, an effect whose mechanism remains poorly understood.

A 2018 systematic review and meta-analysis reached a similar conclusion, pooling data from prospective and cross-sectional studies to show significantly elevated risk of cough, sputum production, wheezing and dyspnoea among marijuana users, while judging the evidence on pulmonary function decline and obstructive lung disease "insufficient" to draw firm conclusions. Earlier work published in the Annals of the American Thoracic Society similarly concluded that regular cannabis smoking causes visible injury to the large airways and chronic bronchitis-type symptoms that subside on cessation, but does not appear to produce the same degree of fixed airflow obstruction associated with cigarette smoking.

The U.S. National Academies of Sciences' influential 2017 consensus report classified the cannabis-COPD relationship cautiously, finding only limited evidence linking occasional cannabis smoking to COPD risk once tobacco use is controlled for, but substantial evidence connecting long-term cannabis smoking to worse respiratory symptoms and more frequent chronic bronchitis episodes. A more recent 2024 state-of-the-art review concluded that cannabis smoke irritates the bronchial tree, is strongly linked to chronic bronchitis, impairs alveolar macrophage antimicrobial function, and is a risk factor for bullous lung disease and spontaneous pneumothorax, while stopping short of establishing a clear association with COPD itself.

Points of Persistent Scientific Uncertainty

Not every dimension of cannabis-related lung harm is settled. The relationship between cannabis smoking and lung cancer remains genuinely contested: a 2025 review noted the link "remains inconclusive," citing conflicting epidemiological evidence despite cannabis smoke sharing carcinogenic constituents, including polycyclic aromatic hydrocarbons, with tobacco smoke. Similarly, moderate cannabis use does not appear to meaningfully impair lung function in several analyses, whereas heavy, prolonged use may cause airflow obstruction without producing the tobacco-characteristic emphysema pattern.

This nuance matters clinically. One widely cited case-control analysis found that tobacco-only smokers, but not cannabis-only smokers, showed significantly elevated odds of COPD compared with non-smokers — yet combined cannabis-and-tobacco use carried a markedly higher risk of both respiratory symptoms and COPD than either substance alone, particularly beyond a lifetime threshold of roughly 50 marijuana cigarettes. This suggests an additive or synergistic harm profile when cannabis and tobacco are co-used, a pattern of considerable relevance given how frequently the two are combined in real-world use.

Public Health Bodies Converge on a Cautionary Message

Public health authorities have already begun translating this accumulating evidence into consumer guidance. The U.S. Centres for Disease Control and Prevention states plainly that smoked cannabis, regardless of the method of administration, "can harm lung tissues and cause scarring and damage to small blood vessels," and that cannabis smoke contains many of the same toxins, irritants and carcinogens found in tobacco smoke. The American Lung Association has gone further, explicitly cautioning the public against smoking marijuana, citing evidence that marijuana smoke injures the cell linings of large airways, suppresses immune defences against infection, and has been associated on CT imaging with greater airway thickening, inflammation and emphysema compared with both non-smokers and tobacco-only smokers.

Why the Molecular Findings Matter for Prevention

What distinguishes the CANUCK study from much of the earlier literature is its move beyond symptom questionnaires and spirometry into airway cell biology. By demonstrating that heavy cannabis exposure alters immune signalling and mucin gene expression in a way that correlates with worse clinical and imaging outcomes, the research offers a plausible biological mechanism for the symptomatic and functional changes long observed in cannabis smokers. This mechanistic layer strengthens the case that respiratory harms from cannabis smoke are not merely coincidental with heavy use but reflect a dose-dependent biological injury pathway, comparable in structure, if not in magnitude, to that seen with tobacco.

As Dr Leung and colleagues note, the findings "underscore the concerns for future respiratory morbidities related to persistent cannabis use," a conclusion with direct relevance for clinicians counselling patients and for policymakers navigating an era of expanding legal cannabis access. With global cannabis consumption continuing to climb amid legalisation trends across North America and elsewhere, researchers argue that larger, longer prospective studies combining spirometry with high-resolution imaging will be essential to fully map the trajectory of cannabis-related lung disease.

Sources:

• Blevins Primeau, A. S. (2025). Cannabis use and its impact on respiratory physiology and lung health. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC12516480/
• Centres for Disease Control and Prevention. (2024, February 25). Cannabis and lung health. https://www.cdc.gov/cannabis/health-effects/lung-health.html
• European Respiratory Journal. (2026, January 28). ERJ podcast January 2026: Cannabis and the lung [Podcast interview with J. Leung]. Eur Respir J, 67, 26E6701. https://doi.org/10.1183/13993003.E6701-2026
• Ghasemiesfe, M., Ravi, D., Vali, M., Korenstein, D., Arjomandi, M., Frank, J., Austin, P. C., & Keyhani, S. (2018). Marijuana use, respiratory symptoms, and pulmonary function. Annals of Internal Medicine. https://pubmed.ncbi.nlm.nih.gov/29971337/
• Ghasemiesfe, M., Barrow, B., Leonard, S., Keyhani, S., & Korenstein, D. (2019). Effects of marijuana smoking on the lung. Annals of the American Thoracic Society. https://pubmed.ncbi.nlm.nih.gov/23802821/
• Leung, C. et al. (2026). The Canadian Users of Cannabis Smoke (CANUCK) study. European Respiratory Journal. https://pubmed.ncbi.nlm.nih.gov/41198398/
• National Academies of Sciences, Engineering, and Medicine. (2017). Respiratory disease. In The health effects of cannabis and cannabinoids: The current state of evidence and recommendations for research (Chapter 7). National Academies Press. https://www.nationalacademies.org/read/24625/chapter/9
• Rezaei, N., et al. (2024). Effects of cannabis smoking on the respiratory system: A state-of-the-art review. Respiratory Medicine. https://www.sciencedirect.com/science/article/pii/S0954611123003827
• American Lung Association. (n.d.). Marijuana and lung health. Retrieved August 11, 2026, from https://www.lung.org/quit-smoking/smoking-facts/health-effects/marijuana-and-lung-health
• Tashkin, D. P. (2016). Effect of cannabis smoking on lung function and respiratory symptoms. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC5072387/
• Cannabis and lung health: Does the bad outweigh the good? [PDF]. https://d-nb.info/1250157714/34
• Cannabis – a rewritten history and its pulmonary implications. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC9923077/

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Published: 06 August 2026

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The federal marijuana rescheduling hearing has closed after twelve days of testimony. The closing witnesses left behind a sobering record. Rather than building a case for marijuana as ordinary medicine, the cannabis rescheduling case exposed a different picture. Witnesses described uncontrolled products, unresolved psychosis risk and a persistent link to organised crime. A recommendation from the presiding judge is due in August. These final days of testimony carry real weight for how the cannabis rescheduling case will ultimately be decided.
Pharmacist Tells Cannabis Rescheduling Case That Safeguards Are Missing
California pharmacist Phil Drum opened the closing stretch of the hearing. He gave a professional’s view of what marijuana lacks. Marijuana carries a long list of adverse effects and drug interactions, he told the court. A pharmacist must catch and disclose these risks before handingover any medicine. State cannabis programmes rarely catch them, he said, and dispensary staff have no clinical training to fill that gap.
Drum also raised concerns about how the products reach consumers. THC edibles are often packaged to resemble familiar snack brands. That blurs the line between a controlled intoxicant and something a child would recognise from a supermarket shelf. He pointed to a human cost too. Veterans who avoid marijuana take their own lives at roughly half the rate of those who use it, he testified.
Recalls told a similar story. Notices for pulled marijuana products often reveal mould, bacteria and pesticides. These recalls also move far slower than an equivalent FDA action would.
Physician Warns of Psychosis and Youth Harm From Cannabis
Emergency physician Dr Karen Randall followed, drawing largely on patients she has treated herself. She dismissed several medical claims made for marijuana. The drug is useless or counterproductive, she said, for conditions such as seizures, Alzheimer’s disease, glaucoma, sleep apnoea and nausea.
Randall described marijuana as a hallucinogen with the power to derail a young life. Her list of consequences ran long: academic collapse, anxiety, depression, cannabinoid hyperemesis syndrome and cannabis use disorder. She also cited lasting changes to brain pathways, psychosis, schizophrenia and suicide. Treatment beds remain scarce, she said. Young people struggle to find care for cannabis addiction, particularly when psychosis or schizophrenia is also present.
She flagged a further blind spot. Marijuana does not appear in prescription drug monitoring programmes. A treating doctor, as a result, often cannot tell whether a patient is using it at all.
Yale Psychiatrist Links Marijuana to Psychosis Risk
Counsel for Idaho, Indiana and Nebraska opened their case with a point of history. It is one that has echoed through every marijuana rescheduling hearing to date. Marijuana has faced review for rescheduling nine times before. Each review reached the same conclusion: no currently accepted medical use and a high potential for abuse. To support that history, the states called Dr Deepak D’Souza, a Yale professor of psychiatry and a staff psychiatrist with the Department of Veterans Affairs.
D’Souza explained how THC overwhelms the brain’s cannabinoid receptors. The brain, he said, sheds receptors in response. He also explained why double blind, placebo controlled studies matter most, and why they rarely support the government’s benefit claims.
On the substance, D’Souza was direct. He called marijuana a cause of psychosis and schizophrenia, a link studied for two centuries. He placed its abuse potential above that of LSD or MDMA. Withdrawal, he testified, can be disturbing enough to force people back into use or into hospital care. Exposure during pregnancy is absolutely dangerous to the developing child, he added.
The pain research the government leans on rests on small samples and subjective outcomes. It strains credulity, he said, that a single drug would work across sixty separate conditions.
Sheriff Testifies That Legalisation Has Not Reduced Crime
The final witness was Sheriff William Honsal of Humboldt County, California. He described what has actually happened where marijuana is grown at scale. Honsal walked the hearing through the state’s long experiment with legalisation, from Proposition 215 through Proposition 64. Criminal activity, he said, has trailed the industry throughout: labour trafficking, drug trafficking, environmental crimes, robbery and assault. In several cases, he added, homicide too.
Honsal explained why the black market persists. Wholesale prices have collapsed under the weight of testing, taxation and compliance costs. Licensed growers, as a result, sit at a structural disadvantage against illegal operators who carry none of those burdens. Organised crime groups have used that gap as a foothold rather than a casualty of legalisation. Honsal named MS13, the Sinaloa Cartel, and Chinese and Bulgarian networks operating in his county.
Asked directly whether legalisation has reduced crime, Honsal answered simply that it has not. He pointed to his own jail as evidence. The share of inmates with prior mental health issues has climbed from around 20 percent to about 50 percent over the past twenty years.
What the Cannabis Rescheduling Case Means Going Forward
Taken together, the testimony from these final days complicates the picture the government set out to build. Pharmacology, clinical experience, psychiatric research and law enforcement all pointed towards the same conclusion. The safeguards that define genuine medicine are largely absent from the marijuana marketplace, and the social costs of legalisation have not eased with time.
A recommendation is due in August. Whichever way the decision falls, the record built during the cannabis rescheduling case will likely shape this debate for years to come. (Source: WRD News)

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Published: 22 July 2026

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A major global review has linked commercial cannabis legalisation to rising rates of addiction, psychiatric hospital admissions and cannabis use disorder. Researchers at the University of Bath led the study, published in The Lancet Psychiatry. The team tracked cannabis policy changes in dozens of countries from 2000 to 2025. One number stands out immediately: cannabis induced psychosis hospitalisations in Ontario climbed 220.7 percent after commercial retail expanded. That pattern repeats across every commercialised market the review examined.

Why Commercial Cannabis Legalisation Behaves Differently to Decriminalisation

Professor Tom Freeman led the research with Dr Rachel Lees Thorne. Freeman says policy type matters more than whether change happens at all. Decriminalised countries showed little rise in cannabis use. Tightly controlled legal markets showed little rise too. Commercial cannabis legalisation broke that pattern everywhere researchers looked.

Portugal and Czechia removed criminal penalties for possession years ago. Neither country saw a meaningful jump in use. Some regions even recorded falling hospital admissions for cannabis psychosis during periods of lower penalties.

That picture isn’t universal, though. A separate analysis covered ten policy changes across eight countries. Six of seven countries that lowered penalties, Bulgaria among them, saw a small rise in adolescent use. Larger, more rigorous studies found no consistent effect overall. Both results sit in the evidence base. Neither cancels the other out.

Commercial Cannabis Legalisation in Canada and the United States

Canada legalised non-medical cannabis nationally in 2018. Past year use among adults rose from 22 percent before legalisation to 27 percent by 2020, then levelled off near 26 percent. Ontario data tells a sharper story. Emergency visits and hospitalisations for cannabis induced psychosis rose 220.7 percent between 2014 and 2021, climbing from 0.29 to 0.93 cases per 100,000 people. The sharpest rise followed the expansion of commercial retail stores and higher potency products, not the legal change itself.

The United States tells a similar story under commercial cannabis legalisation. Most states adopted a commercial model with few limits on potency, pricing or marketing. Daily cannabis consumers in the US now outnumber daily alcohol consumers, according to Freeman. Colorado hospital data showed psychosis related visits climbing alongside dispensary numbers. Hospitalisations for psychosis rose from 28.0 to 32.3 per 100,000 people, and cases combining psychosis with cannabis use disorder more than doubled, from 3.4 to 8.5 per 100,000.

Commercial pressure keeps pushing potency higher too. Profit driven retailers sell flower and concentrates carrying far higher THC levels than tightly regulated schemes allow. Higher potency use consistently raises the risk of both addiction and psychotic illness.

Hospital Visits Are Not the Same as Population Wide Rates

This distinction matters, and it gets blurred often. Researchers did not find consistent evidence that legalisation changes the underlying prevalence of psychotic disorders across a population. What they did find was a rise in hospital admissions for psychosis, and for psychosis alongside cannabis use disorder, strongest in the most commercialised regions.

That finding still carries real weight. More people are showing up in emergency rooms. More psychiatric beds are filling with cannabis related crises. But it’s a claim about strain on health services, not a settled claim about how many people develop psychotic illness overall. Keeping that distinction clear protects the argument rather than weakening it.

Uruguay Shows a Different Path, Backed by Thin Evidence

Uruguay avoided commercial cannabis legalisation entirely. Adults access cannabis only through registered home growing, nonprofit social clubs, or pharmacies. Government sets price, product range and potency in pharmacies, currently capped at 20 percent THC.

Results compared with neighbouring Chile look encouraging. One study found no significant rise in past year or past month use following legalisation. A second study found an actual drop in past year and past month use among 12 to 17 year olds, plus a meaningful fall in high risk use among 12 to 21 year olds.

Why the Comparison Doesn’t Settle the Question

Two published studies cover this policy, and both focus only on adolescents and young adults. Both track a short window following the 2017 rollout of pharmacy sales. Bath researchers say evidence is still needed for adults older than 21, for psychiatric outcomes, and over a longer follow up period. Uruguay hasn’t visibly harmed teenagers so far. Nobody yet knows what fifteen or twenty years of a stable legal market does to adults, addiction rates or psychiatric illness.

Uruguay’s safety, so far as it holds, depends on government suppressing every commercial incentive that reshaped Canada and the US: advertising, potency competition, retail density, price cuts. Thailand shows what happens when that effort stops.

Thailand Shows How Fast a Legal Market Can Escalate

Thailand removed cannabis from its controlled substances list in 2022, and a fast slide toward commercial cannabis legalisation followed almost immediately. Dispensaries multiplied within months. Inpatient cases of cannabis induced psychotic disorder rose from 477 in 2017 to 2,713 in 2023, according to Thailand’s National Health Security Office, a nearly fivefold increase. Cannabis poisoning cases climbed from 39 to 637 over the same period, and acute intoxications rose from 76 to 1,039.

Facing mounting concern, Thailand’s government reversed course in June 2025. The country now restricts cannabis to medical use only and requires a medical certificate for purchase. Few countries have tested how quickly a legal market can escalate, or how hard it is to walk back once demand and retail infrastructure take hold.

What Commercial Cannabis Legalisation Means for the UK

Cannabis remains a Class B controlled drug in the UK, carrying a maximum penalty of five years in prison for possession. A 2025 report from the London Drugs Commission, commissioned by the Mayor of London, proposed decriminalising possession for recreational use. It also called for shifting enforcement resources toward healthcare.

Robust studies found little consistent link between decriminalisation and use, and the review draws a clear line between that finding and commercial cannabis legalisation, which behaves very differently. Individual country studies tell a messier story, with some, including parts of Europe, recording small increases after similar reforms. Decriminalisation changes the signal a drug sends. It reduces the friction that currently suppresses demand. Portugal shows this often becomes a first step rather than a stable endpoint.

The Only Model With No Downside Risk

Cannabis policy is shifting fast across dozens of countries right now. Every jurisdiction that loosened its rules saw cannabis embed itself further into daily life. The differences between these models come down to speed, not safety. Commercial cannabis legalisation remains the fastest route to harm. It isn’t the only route, though, and researchers have only tested the safest alternative on record on teenagers, over a few short years.

Prevention and demand reduction carry no built in path toward addiction or rising psychiatric admissions. Policy debates that focus only on regulating a cannabis market well skip past the more important question: should cannabis become a normal consumer product at all?

(Source: WRD News)