Marijuana as an alternative treatment for glaucoma
Is cannabis (otherwise known as ‘marijuana’ and a host of other terms) a treatment for glaucoma? Medicinal cannabis is often presented as an alternative treatment for glaucoma. Although there is evidence that cannabis lowers intraocular pressure, its role as a viable glaucoma therapy is limited by a short duration of action, psychotropic effects, and possible tachyphylaxis. Let’s take a look.
What is cannabis?
This word ‘cannabis’ refers to a genus (or family) of plants best known for producing a family of compounds known as ‘cannabinoids’.
The most common variety is Cannabis sativa, a tall plant with long thin fibrous leaves, originally cultivated for fibre and animal feed. Yes, historically its main historical use was for fibre: the first pair of Levi’s was made from hemp (the fibre found in its leaves). It wasn’t common knowledge that this plant had psychoactive properties, perhaps because the early fibrous varieties contained little of the active component. It is the variety which is, however, rich in THC.
The other common variety which is also known for its ‘entertainment’ qualities, is Cannabis indica, a subspecies of cannabis sativa, which is a shorter and bushier plant. This is what is used to make hashish, and is rich in CBN or cannabinol. Cannabis ruderalis originated in Central Russia, and is very fast growing.
Let’s keep the figure and add the figure legend:
“Effect of cannabis inhalation on intraocular pressure (IOP). Reference: Cannabinoids for treatment of glaucoma. G D Novack. Curr Opin Ophthalmol 2016;27:146-50.”
While cannabis has more than 480 chemical constituents, it is only the 66 compounds made of carbon, hydrogen and oxygen that are known as cannabinoids that are of interest to us here. Of the 66 cannabinoids that are known, two are of particular interest. The first is THC (∆-9 tetrahydrocannabinol), which is the main psychotic agent. Nearly all research to date has been done on this. Interestingly, it has been found that the THC concentration of marijuana (one of the many alternative names for this plant) has increased from ~3% in 1970s to ~20% today.
The other commonly known cannabinoid is cannabidiol (CBD). CBD has no ‘elevating’ psychoactive effects, but is thought to reduce anxiety. The cannabinoid profile varies according to the species of cannabis plant, the way it is grown, and which part of the plant is harvested.
Cannabis and intraocular pressure
In 1971 it was discovered that smoking cannabis can lower intraocular pressure. To investigate the effect of cannabis on the human visual system, eleven healthy subjects underwent comprehensive eye exams before and after smoking two grams of cannabis. Unexpectedly, the subjects were fou