In 1973 the New England Journal of Medicine reported that smoked cannabis widened the airways of healthy young men, while tobacco smoke narrowed them. Small studies in people with asthma in 1974–75 found the same short-term effect. This leaves a real contradiction: THC relieves airway narrowing in the short term, but smoking it causes chronic bronchitis symptoms.
Nothing here is medical advice, and nothing here is a reason to change or skip asthma treatment. Asthma can be fatal, and deaths are often linked to under-use of preventer therapy. Inhaled corticosteroids are the treatment that prevents attacks. In Thailand cannabis flower is a controlled herb dispensed only on prescription form PT 33.
1. The 1973 Finding and What Followed
The 1973 study (Tashkin, Shapiro and Frank, NEJM) tested 32 healthy young men, not asthma patients. Smoked marijuana raised airway conductance, which means the airways widened. The effect peaked at about 15 minutes and was still present at 60 minutes. Tobacco smoking, and deep-breathing manoeuvres that copied the way people smoke marijuana, did the opposite. Oral THC also widened the airways, but more weakly.
Tests in people with asthma came later:
- 1974 (Am Rev Respir Dis): 10 adults with stable asthma. Smoked 2% marijuana and oral THC both widened the airways compared with placebo.
- 1975 (Am Rev Respir Dis): 8 adults with asthma. Smoked 2% marijuana quickly reversed airway narrowing that had been triggered in the lab with methacholine or exercise.
Both were small, short-term laboratory studies, not treatment trials. Figures often quoted online, such as “2.6% THC reduced ventilatory obstruction by 38% and increased airway conductance by 44%”, could not be traced to any of these papers and are left out here.
The acute effect has held up. A 2007 systematic review (Tetrault et al., Arch Intern Med) found that 11 of 12 challenge studies showed bronchodilation after short-term marijuana use. A 2016 structured review (Ribeiro and Ind) reports that three of those studies measured FEV1 increases of 150–250 mL above baseline.
So the short-term pharmacology is clear: THC is a bronchodilator. That is an unusual property for a smoked plant material.
2. The Contradiction, Stated Plainly
The same research also shows what chronic cannabis smoking does to the airways, and it is not harmless.
In Tetrault’s review, every study that looked at long-term smoking and respiratory symptoms found more symptoms in smokers. Ribeiro and Ind report that eight of nine studies measuring symptoms found more respiratory complaints in chronic cannabis smokers, with odds ratios up to 2.98 compared with non-smokers. The complaints were cough, sputum, shortness of breath and wheeze, a pattern similar to tobacco smokers. Studies also show small increases in airway resistance. Tashkin’s 2013 review describes visible and microscopic injury to the large airways in regular smokers.
Put together: smoking cannabis opens the airways in the short term and inflames them over time. Asthma is a disease of chronic airway inflammation with episodes of narrowing. For a person with asthma, smoked cannabis would relieve the symptom while adding to the cause.
The practical answer is to avoid smoke. Vaporising heats the material without burning it, and reviews suggest it may reduce respiratory complications. The evidence for this is limited, and vaporising is not proven safe.
3. Cannabis and COPD: The Evidence Is Mixed
| Finding | Tobacco smokers | Cannabis smokers |
|---|---|---|
| Chronic bronchitis symptoms | Increased | Increased |
| Macroscopic emphysema on CT (Aldington 2007) | 16% (tobacco ± cannabis) | 1.3% (1 of 75, cannabis only) |
| Emphysema on CT (Murtha 2022) | 67% (tobacco only) | 75% (most also smoked tobacco); difference not statistically significant |
| Airflow obstruction / COPD | Develops | Evidence mixed |
| FEV1/FVC | Declines | Most large cohorts: no faster decline, sometimes higher FVC; Aldington 2007: dose-related fall in FEV1/FVC |
Most large cohort studies have not found faster decline in FEV1/FVC in cannabis-only smokers. Some found higher FVC, the opposite direction from tobacco. Tashkin’s 2013 review concludes that no clear link to COPD has been established.
Other data point the other way:
- Aldington 2007 (Thorax, New Zealand) found little emphysema in cannabis-only smokers. The same study found a dose-related airflow obstruction: for airflow measures, one joint had about the same effect as 2.5–5 tobacco cigarettes.
- Murtha 2022 (Radiology, Canada) found emphysema in 75% of marijuana smokers’ CT scans. It also found airway changes more often than in tobacco-only smokers: bronchial wall thickening, bronchiectasis and mucus plugging. Most of the marijuana smokers also smoked tobacco, and the authors could not prove that cannabis caused the emphysema.
- Case reports describe bullous lung disease (large air spaces in the lung) in heavy cannabis smokers.
The question is not settled.
Several factors explain why studies disagree. Cannabis smokers usually burn fewer joints a day than tobacco smokers burn cigarettes, and THC itself widens the airways. But cannabis smokers also inhale more deeply and hold their breath longer, which leaves more tar and particles in the lungs per joint. And many people mix cannabis with tobacco (spliffs, a common practice in many countries). For them the tobacco risk, including emphysema and COPD, is fully present.
None of this makes smoking safe. At best, cannabis smoke harms the lungs differently from tobacco smoke. Chronic bronchitis is a disease in its own right, and the emphysema question is still open.
4. What This Means for Emphysema
Some cannabis writing presents cannabis as a help for emphysema. No clinical trial has shown this.
Lung tissue destroyed by emphysema does not regenerate. A bronchodilator can improve airflow through the airways that remain, but it does nothing for destroyed air sacs.
For someone who already has emphysema, usually after decades of tobacco, inhaling any smoke is a poor idea, whatever its bronchodilator content. No trial has compared cannabinoids by any route with established bronchodilators.
5. What Asthma Treatment Actually Requires
Asthma is not managed with bronchodilation alone. This is the main safety point.
Modern asthma care is built on inhaled corticosteroids, which treat the underlying airway inflammation, with bronchodilators for quick symptom relief. Relying on short-acting relievers while under-using preventer therapy is a recognised and dangerous pattern. The patient feels treated while the inflammation that causes attacks goes untreated. Severe and fatal attacks are concentrated in this group.
A drug that widens the airways without treating inflammation fits that pattern exactly. If it is smoked, it adds inflammation of its own.
Inhaled cannabis carries one more risk: cannabis material can be contaminated with moulds and bacteria. This is especially serious for people with weak immune systems or damaged lungs. Topic 1359/49 covers cannabis quality and contamination.
6. The Honest Summary
Established: THC is a short-term bronchodilator. This was shown in healthy volunteers in 1973 and in small asthma studies in 1974–75, and has been confirmed in later challenge studies.
Also established: chronic cannabis smoking increases cough, sputum, wheeze and breathlessness.
Not settled: whether cannabis-only smoking causes emphysema or COPD. Most cohort data are reassuring, but some CT and cohort studies are not. Mixing with tobacco brings back the full tobacco risk.
What follows: smoking is the wrong route for anyone with a lung condition.
Not established: that cannabis or any cannabinoid is an effective treatment for asthma or COPD. No cannabinoid is licensed for either, no guideline recommends one, and there are no trials comparable to those for existing drugs.
7. In Thailand
Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568, issued on 26 June 2025. Flower may be dispensed only with a prescription on form PT 33 from an authorised practitioner. Each prescription covers no more than 30 days and is single-use, with no refills. Flower that is sold must come from a GACP-certified cultivation site. Online sale and advertising of flower are prohibited, and recreational use is not permitted.
Asthma and COPD are not among the indications in the Department of Medical Services guidance on medical cannabis. Standard inhaled treatments for both are effective and widely available.
For discussion: what have you read or been told about vaporising versus smoking and respiratory symptoms such as cough, sputum or wheeze? Do you know of good studies we have missed? If you have asthma, COPD or any lung condition, talk to your doctor before considering any form of cannabis, and do not change your prescribed inhalers on your own.
References
- Tashkin DP, Shapiro BJ, Frank IM. Acute pulmonary physiologic effects of smoked marijuana and oral Δ9-tetrahydrocannabinol in healthy young men. N Engl J Med 1973;289(7):336–341. https://doi.org/10.1056/NEJM197308162890702
- Tashkin DP, Shapiro BJ, Frank IM. Acute effects of smoked marijuana and oral Δ9-tetrahydrocannabinol on specific airway conductance in asthmatic subjects. Am Rev Respir Dis 1974;109(4):420–428. https://doi.org/10.1164/arrd.1974.109.4.420
- Tashkin DP, Shapiro BJ, Lee YE, Harper CE. Effects of smoked marijuana in experimentally induced asthma. Am Rev Respir Dis 1975;112(3):377–386. pubmed.ncbi.nlm.nih.gov
- Tetrault JM, et al. Effects of marijuana smoking on pulmonary function and respiratory complications: a systematic review. Arch Intern Med 2007;167(3):221–228. pubmed.ncbi.nlm.nih.gov
- Aldington S, et al. Effects of cannabis on pulmonary structure, function and symptoms. Thorax 2007;62(12):1058–1063. Effects of cannabis on pulmonary structure, function and symptoms - PMC
- Tashkin DP. Effects of marijuana smoking on the lung. Ann Am Thorac Soc 2013;10(3):239–247. https://doi.org/10.1513/AnnalsATS.201212-127FR
- Ribeiro LIG, Ind PW. Effect of cannabis smoking on lung function and respiratory symptoms: a structured literature review. NPJ Prim Care Respir Med 2016;26:16071. Effect of cannabis smoking on lung function and respiratory symptoms: a structured literature review - PMC
- Murtha L, et al. Chest CT findings in marijuana smokers. Radiology, published online November 2022. Summary: Emphysema In Marijuana Smokers | RSNA
This article is general educational information, not medical advice. Never reduce or stop asthma preventer therapy. It is what prevents attacks.