Controlling intraocular pressure (IOP) is the main goal of glaucoma treatment; it preserves comfort and slows the progression of optic neuropathy. While various medical and surgical treatments exist, none are always effective. Delta-9 tetrahydrocannabinol (THC), derived from the cannabis plant (ie, marijuana), is known to lower IOP. The present study evaluated the efficacy of a 2% THC ophthalmic solution in lowering IOP and aqueous humor flow rate (AHFR) in clinically normal dogs. Sixteen client-owned dogs were randomly assigned to 1 of 2 treatment groups.

Either 2% THC or a control solution was instilled in 1 eye q12h for 9 doses. IOPs and AHFR were measured in the morning and evening at baseline and after treatment completion. Mean morning IOPs were 15.86 mm HG at baseline, 12.54 mm Hg in the THC group, and 13.88 mm HG in the control group; mean evening IOPs were 13.69 mm HG, 11.69 mm HG, and 12.13 mm HG, respectively. THC reduced morning IOP by 21% and evening IOP by 15%; however, there was no significant reduction in AHFR. These results indicated that THC’s effect on IOP is similar to that of conventional topical glaucoma therapies. The magnitude of the IOP decrease varied between dogs: 4 exhibited a >30% decrease, 6 had decreases between 5% and 30%, and 6 had slight or no decrease. Further study is warranted to determine if the effect is more robust in dogs with glaucoma.

Source
Effects of a topically applied 2% delta-9-tetrahydrocannabinol ophthalmic solution on intraocular pressure and aqueous humor flow rate in clinically normal dogs. Fischer KM, Ward DA, Hendrix DVH. AM J VET RES 74:275-280, 2013.