Activation of CB1 receptor is known to affect IOP. It is present both in ciliary bodies along with GPR18 and in the trabecular network. Exogenous cannabinoids in clinical studies have demonstrated a 10–30% reduction in IOP 2–4 h after dosing with a dose-dependent relationship between the amount of medication and lowering of IOP; however, dose increases did not increase the duration of action. GPR18 agonists also lower IOP. CB1 antagonist also lowers IOP under some conditions, making the likelihood of glaucoma being an endocannabinoid deficiency syndrome exceedingly small. The components of the ECS are extensively expressed in the retina as well and are known to function in the typical feedback inhibition fashion, specifically on GABAergic and glutamatergic neurons; however, their exact role is still unknown. Animal models of retinitis pigmentosa showed neuroprotective effects of the ECS, but activation of CB2 in a macular degeneration model actually increased inflammation.