Asthma is the shortness of breath and wheezing caused by spasms of the bronchial tubes, overproduction of mucus, and swelling of the mucous membranes. Asthma kills more than 4,000 Americans each year.
Clinical research shows that THC acts as a bronchial dilator, clearing blocked air passageways and allowing free breathing.,  In one study, marijuana, “caused an immediate reversal of exercise-induced asthma and hyperinflation.” Numerous cases of asthma have been treated successfully with both natural and synthetic THC. In one report, a young woman used marijuana with her doctor’s approval. Over the course of several years her attacks were almost completely cured with low doses of inhaled cannabis smoke.
Some asthmatics who have found relief through the use of synthetic THC often voice a preference for natural cannabis over Marinol. Marinol is said to be less effective than natural cannabis and has far greater psychoactive properties.
Alternative methods of administration have been recommended by the Institute of Medicine  and other medical authorities. Plans for a noncombusting THC inhaler received attention for many years, yet designers have failed to produce a workable prototype.
“Experiments examining the antiasthmatic effects of THC or cannabis date mainly from the 1970s, and are acute studies. The effects of a marijuana cigarette (2 percent THC) or oral THC (15 mg), respectively approximately correspond to those obtained with therapeutic does of common bronchodilators drugs (salbutamol, isoprenaline). Following inhalation, the effect lasts about two hours. Since inhalation of cannabis products may irritate the mucous membranes, oral administration or another alternative delivery system would be preferable. Very few patients developed brochoconstriction after inhalation of THC.” 
Related sections: Immune Responses, Muscle Spasms, Psychoactivity, Respiratory Disease, Smoking Methods, Stress Reduction.
 Grinspoon, “Marijuana and asthma.” The Forbidden Medicine Website, www.rxmarijuana.com
 National Academy of Science, 1982
 “Therapeutic possibilities in cannabinoids,” Editorial, The Lancet, pp. 667-669, March 22, 1975
 Tashkin, Shapiro, Lee, and Harper, “Effects of smoked marijuana in experimentally induced asthma.” American Review of Respiratory Disease, Vol. 112, 1975
 Letters, High Times, No. 273, May, 1998
 Institute of Medicine, Marijuana and Medicine: Assessing the Science Base. Washington DC: National Academy Press, 1999
 Geiringer, “An overview of the human research studies on medical use of marijuana.” CANORML, 1994, www.norml.org/canorml/
 Grotenhermen, Russo. Cannabis and Cannabinoids: Pharmacology, Toxicology, and Therapeutic Potential. New York: The Hawthorn Integrative Healing Press, 2002, Grotenhermen, “Review of Therapeutic Effects.” Chapter 11, p. 130
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