The Cannabis sativa plant and its close relatives are used to make hashish and marijuana, which are illicit and psychoactive substances with widespread consumption in society. The major active compound in these substances is tetrahydrocannabinol (THC), although marijuana is a heterogeneous mixture of chemicals. Many ophthalmologists who care for glaucoma patients have been asked by their patients about the ocular hypotensive effect of marijuana and its potential for therapy of glaucoma.
Ocular effects of marijuana include decrease in pupillary size with preservation of normal responsiveness to light, decrease in tear secretion, conjunctival hyper-emia, and decrease in IOP.54 In one study, IOP was reduced in 9 of 11 normal subjects, with a mean decrease of 25%, 1 hour after pipe smoking 2 g of marijuana containing approximately 18 mg THC.55 When patients with ocular hypertension or glaucoma were tested, 7 of 11 showed a reduction in IOP averaging 30%.56 In 31 glaucomatous eyes in 18 patients with different types of glaucoma, the maximum decrease in IOP was 6.6mmHg (~23% less than baseline) 90 minutes after inhalation of marijuana (figure 9.2).57 In various human studies, the maximal reduction in IOP occurred at 60 to 90 minutes, and the duration of the decrease was approximately 4 to 5 hours, despite various routes of administration.58
On average, smoking marijuana reduces IOP in at least 60% to 65% of users; however, continued use leads to substantial systemic toxic effects.59 In one study, initial decreases in IOP were not sustained over time, and all nine patients enrolled in the study elected to discontinue treatment within 1 to 9 months for various reasons.60
Clinical studies have not clearly defined the effects of marijuana on aqueous dynamics, which may involve a local effect on the eye and a central nervous system effect. However, preclinical testing on animals has shown that the reduction in IOP
Figure 9.2. Effect of marijuana smoking on mean IOP in 31 eyes in 18 patients with different types of glaucoma (blue circles, placebo; orange circles, marijuana). After marijuana inhalation, IOP was significantly reduced, with maximal response at 90 minutes and duration of effect up to 4 hours. Data from Merritt JC, Crawford WJ, Alexander PC, et al. Effect of marihuana on intraocular and blood pressure in glaucoma. Ophthalmology. 1980;87:222-228.
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