British researchers investigated the effects of THC and CBD in six patients with ocular hypertension or glaucoma. In a four-way crossover study participants received 5 mg THC, 20 mg CBD, 40 mg CBD, or a placebo. The substances were applied to the mucosa of the mouth.

Two hours after sublingual administration of THC, the intraocular pressure (IOP) was significantly lower than after placebo (on average 23.5 mm Hg versus 27.3 mm Hg). The pressure returned to baseline level after the 4-hour measurement. CBD administration did not reduce the intraocular pressure, but the highest dose caused a small increase in pressure after four hours. Visual acuity were not significantly changed. One patient experienced a transient and mild paniclike reaction after THC administration. Researchers concluded that "a single 5 mg sublingual dose of Delta-9-THC reduced the IOP temporarily and was well tolerated by most patients."

(Source: Tomida I, Azuara-Blanco A, House H, Flint M, Pertwee RG, Robson PJ. Effect of sublingual application of cannabinoids on intraocular pressure: a pilot study. J Glaucoma. 2006 Oct;15(5):349-353.)