Glaucoma is a common cause of blindness.
Studies in the early 1970s showed that marijuana, when smoked, lowered intraocular pressure (IOP) in people with normal pressure and those with glaucoma. In an effort to determine whether marijuana, or drugs derived from marijuana, might be effective as a glaucoma treatment, the National Eye Institute (NEI) supported research studies beginning in 1978. These studies demonstrated that some derivatives of marijuana transiently lowered IOP when administered orally, intravenously, or by smoking, but not when topically applied to the eye.
However, none of these studies demonstrated that marijuana -- or any of its components -- could lower IOP as effectively as drugs already on the market. In addition, some potentially serious side effects were noted, including an increased heart rate and a decrease in blood pressure in studies using smoked marijuana.
- Advocates of medicinal marijuana cite evidence that hemp products can lower intraocular pressure (IOP) in people with glaucoma. However, these products are less effective than safer and more available medicines. Most research regarding marijuana use took place before some current medications with fewer side effects were available. The high dose of marijuana necessary to produce a clinically relevant effect on IOP in the short term requires constant inhalation, as much as every three hours. The number of significant side effects generated by long-term oral use of marijuana or long-term inhalation of marijuana smoke make marijuana a poor choice in the treatment of glaucoma. 
- Medical Marijuana Glaucoma Research Foundation