THE EVIDENCE SHOWS that enthusiasm for medical marijuana has outpaced the scientific evidence in favour of it

With very little fanfare, Health Canada has changed the law and now allows physicians to prescribe marijuana to patients for certain medical conditions. On April 1, Health Canada implemented its Marihuana for Medical Purposes Regulations.

Why it chose April Fool’s Day and why it spells marijuana with an “h” are still unclear to me. Nevertheless, Canadians can now go to their doctor and ask for a prescription of dried cannabis for medical purposes. But just because you can does not mean that you should.

Of the many purported health benefits of marijuana, the most-often repeated is its ability to prevent glaucoma. Unfortunately, the evidence to support that claim is fairly limited. First, there are no major trials on the subject; and most of the evidence is from small studies of only a handful of patients.

Glaucoma is caused by high pressure within the eye, and these studies show that delta-9-tetrahydrocannabinol (THC, the active ingredient in marijuana) has some ability to lower intra-ocular pressure. However, this dip in pressure lasts only a few hours. Whether it has longterm benefits is unknown. Also, it appears to be very dose-dependent. Whereas 5 mg showed a temporary decrease in pressure, the 20-mg dose showed no effect, and the 40-mg dose actually in- creased pressure in the eye.

As a result, the Canadian Ophthalmological Society does not support the use of marijuana to treat, or prevent, glaucoma, citing its short duration of action, its psychotropic side effects and the fact that much better treatments for glaucoma already exist — namely a variety of eye drops and laser surgery.

The other main claim to fame for marijuana is its use to prevent chemotherapy-induced nausea and vomiting (CINV).

Synthetic cannabinoids like nabilone and dronabinol have been available and in use for some time. Although they have some antiemetic effect and are superior to a placebo sugar pill, there is no clear evidence that they are better than other medications to treat nausea.

Bear in mind that most of the evidence relates to these medications in pill form, not to the smoking of marijuana, for which little data exist.

So while there might be some role for cannabinoids when first-line medications like ondansetron are ineffective, their role is fairly limited.

The benefit of marijuana to aide in musculoskeletal pain and other neurological syndromes is also somewhat dubious.

There have been a few studies in patients with multiple sclerosis. A Canadian study published in the Canadian Medical Association Journal in 2012 suggested a benefit to patient-reported symptoms, but not in the objective timed-walk test.

However, this study was unblinded and therefore prone to bias from the placebo effect. Two studies of note, the CAMS and the CUPID study, both published in the Lancet in 2003 and 2013 respectively, showed no benefit to using THC.

Studies in diseases like Huntington’s disease and Tourette’s syndrome have also shown no benefit. The data for epilepsy are conflicting, with some studies suggesting a benefit while other studies suggest an increased epilepsy risk. There are no studies on fibromyalgia or arthritis. A recent review by Dr. Mary-Ann Fitzcharles of the McGill University Health Centre concluded there was no benefit to marijuana in most rheumatological conditions.

Unfortunately, the enthusiasm for medical marijuana has outpaced the science.

Although there might be some role for it, specifically in cancer patients, the evidence is hardly overwhelming. Also, almost all the studies I have mentioned here look at THC in pill form, not in smoking a marijuana cigarette.

Although likely far safer than nicotine cigarettes, smoked marijuana may still have some association with lung disease and cancer. So while smoking marijuana is not nearly as dangerous as heroine or cocaine, and while an argument can be made for its decriminalization, its medical use is very limited.

So to all those proponents and enthusiasts of cannabis, I suggest you write to your member of Parliament and make your case. Leave your family doctor alone. He or she is busy enough as it is.