@woodcutter: I found this article at Medscape. org (education for doctors & nurses): Effects of Smoking Cannabis on Lung Function: Effect of Cannabis on Airflow Obstruction (link might require sign-up).
Cannabis has long been recognized as a bronchodilator. Indeed, newspapers in New Zealand (and presumably many other countries) carried advertisements for imported cannabis cigarettes as a treatment for asthma in the late 1800s.[101] Hence, inhaling cannabis appears to predate inhaled adrenergic bronchodilator therapy by at least half a century.[14] There appears to be no doubt that smoking cannabis does have acute bronchodilator effects: in a recent systematic review, 11 out of 12 studies demonstrated a bronchodilator effect of cannabis.[10] However, this acute bronchodilator effect is modest and does not appear to be sustained with continued use over 6–8 weeks.[15] It has been shown to be of slower onset than salbutamol, which has greater bronchodilator effects at 5 min compared to tetrahydrocannibinol.[16] The potential short-term therapeutic effects also need to be weighed against the adverse effects of increased bronchitis and exacerbations of asthma that have been associated with regular cannabis use. Consequently, cannabis is not currently considered to have a therapeutic role in acute bronchospasm (although this is occasionally claimed by cannabis users to justify their habit) and the acute effects will not be considered further in this article…
In summary, there is currently no convincing evidence that smoking cannabis causes airflow obstruction. This may be surprising and appears to conflict with the consistent evidence for increased resistance to airflow in the large airways…
This supports @gambitking‘s claim of treating asthma, even if not recommended as long-term medical therapy.
It’s a similar situation with glaucoma (ocular hypertension): Should You Be Smoking Marijuana To Treat Your Glaucoma? (from glaucoma. org)
…although marijuana can lower the eye pressure, recommending this drug in any form for the treatment of glaucoma at the present time does not make sense given its side effects and short duration of action, coupled with a lack of evidence that its use alters the course of glaucoma.