Medical cannabis studies for the treatment of glaucoma date back to the 1970s, and back then studies showed weed smoking lowered IOP in glaucoma patients. Like many other diseases, glaucoma may be treated with medical marijuana due to the presence of cannabinoid receptors in the ciliary body, which regulate aqueous humor production. It has been shown that CB1 receptors are involved in IOP control, and THC administration may lower aqueous humor production.
Even synthetic CB1 receptor agonists are shown to reduce IOP in normal and glaucomatous animal eyes, possibly by decreasing aqueous flow. These results have given the basis of developing marijuana-based anti-glaucoma drugs.
In uncontrolled studies, smoking whole-plant cannabis, as well as oral and intravenous administration of THC have all lowered IOP in glaucoma patients. The IOP-lowering effectof THC has been reported by 60-65% of glaucoma and non-glaucoma subjects (reduction by approximately 25%, compared to pre-treatment IOP value). The change in IOP values appeared to have a dose-response relationship. Meaning, increasing the cannabinoid dose was associated with greater reduction in IOP value. The IOP-lowering benefit might be due to the peripheral vasodilatory effects of cannabinoids and decreased pressure in ciliary body vasculatures. The treatment benefit lasted only for 3-4 hours, and higher doses did not increase the duration of the effects.
Irrespective of route of administration, cannabis does lower IOP. However, the benefit lasts for about 3-4 hours, which is a major limitation of marijuana use for glaucoma treatment. Typically, glaucoma symptoms need to be under control 24/7; if you opt for medical cannabis, you need to administer cannabis at least 6-8 times a day to maintain optimal IOP, round the clock. Regular, repeated doses are not always possible in most patients due to the psychoactive effects, and missed doses are common.
To avoid these issues, researchers tried to employ topical eye drops with a formulation of THC. However, the occurrence of eye irritation and its stimulation of tears washed away the medication and prevented drug absorption. As the role of ocular cannabinoid receptors has been implicated in glaucoma, it is still worthy to pursue drug development in topical eye drop form with an effective drug delivery system without causing systemic adverse events.
It is NOT the limited efficacy of cannabis but rather prohibition that is preventing the development of cannabis-based topical application eye drops. In the United States, the chance of conducting this type of research is remote due to federal restrictions. We hope this situation will change in the near future.
You can read the other half of the article on the Marijuana Times: http://bit.ly/2blRi9X
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