Physicians unlikely to embrace marijuana as medicine

December 01, 2007

Keith Humphreys, OpEd, San Francisco Chronicle

It wasn't just women with breast cancer who were excited last month when scientists at California Pacific Medical Center Research Institute showed that a compound found in marijuana may be able to block the growth of aggressive tumors. This finding also cheered activists who hope that mainstream medicine will soon embrace marijuana as a treatment. For a range of reasons, that's extremely unlikely.

Effective medicines can of course be derived from plants. Digoxin from foxglove, atropine from belladonna and quinine from cinchona are only a few examples. The marijuana plant likewise contains potentially therapeutic compounds known as cannabinoids, one of which, cannabidiol, was examined in the breast cancer study. Other research has examined tetrahydrocannabinol (THC) - the cannabinoid in marijuana that is primarily responsible for the plant's psychoactive effects (e.g., feeling "high," hallucinations, changes in mood). THC has been shown to benefit at least some patients with a range of problems, including chemotherapy-induced nausea and the tremors and muscles spasms associated with multiple sclerosis.

Nonetheless, only a minority of physicians harbor great enthusiasm for prescribing marijuana cigarettes. Indeed, a survey of almost a thousand physicians by Brown University researchers showed that doctors are significantly less supportive of medical marijuana than is the general public.

Older members of the field remember vividly the era when most physicians smoked tobacco cigarettes and cheerfully rated Camel their favorite brand. The tobacco industry built on this foundation with deceptive advertisements linking doctors with smoking in the public mind (currently on exhibit at the UC San Francisco library on 530 Parnassus Ave.), which damaged medicine's credibility.

These bitter historical experiences, supplemented by decades of subsequent research evidence that smoke inhalation of all forms (even wood smoke) can cause acute and long-term respiratory damage, make many physicians wary of recommending a smoked medicine. A smoked plant has the further disadvantage from a medical perspective of not being pure (e.g., what if the plant had been sprayed with pesticide?) or of a standardized dose. This exposes the patient to risk of side effects, and the physician to risk of malpractice.

As the California Pacific research team noted, for example, obtaining the correct dose of cannabidiol through smoking marijuana would be virtually impossible. It would also of course cause THC's psychoactive effects (cannabidiol is not psychoactive), which some patients find aversive. Will all the therapeutic components of marijuana one day be available in pure, standardized forms that can be safely administered without combustion?

Liquid THC, known as dronabinol, has been available by prescription for years and has some evidence of effectiveness, but its slow absorption after ingestion makes it unappealing to some patients.

Several companies are working to make a dronabinol mist that could be taken in a standardized dose with an inhaler, such as is done with medicines for asthma. An alternative approach, being tested at UCSF, is to heat marijuana in a vaporizer so that THC can be inhaled without the carcinogens found in marijuana cigarettes.

If these technological breakthroughs are achieved, some physicians will become more comfortable with prescribing THC. But others will have the opposite reaction because purified, inhalable (and therefore fast-acting) THC could carry more addictive risk than marijuana itself.

Addiction medicine specialists are aware of this possibility, which may be why the Brown University survey showed that they were less sanguine about medical marijuana than doctors in any other specialty.

In general, as plant-based compounds that can produce dependence are processed and purified (e.g., from coca leaf to cocaine or opium poppy to morphine) or are administered through a more rapid, efficient route (e.g., from ingesting to smoking), their power to produce addiction increases.

In other words, the very dosing technology that could makes THC more pure and potent as a medicine may also make it more likely to produce dangerous dependence. Unless further research reveals a way to cut that Gordian knot, THC will probably remain a bit player in mainstream medicine practice.

Keith Humphreys is a professor of psychiatry at Stanford Medical School.



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