Scientific exploration of its use for glaucoma dates back to 1971, when Hepler and Frank documented a 25% to 30% reduction in IOP for 3 to 4 hours in volunteers who smoked it. In 1980, Merritt et al. repeated the finding in a placebo-controlled trial.
Other researchers found that oral ingestion and sublingual administration of a whole plant extract of THC also reduced IOP for 2 to 4 hours. But topical THC administered in a light mineral oil had no effect compared with controls.
And one study found that taking THC daily resulted in only a transitory effect, with all patients electing to discontinue within 9 months.
There are no long-term studies on the use of cannabis for glaucoma, Dr. Radhakrishnan said.
Case reports (published or anecdotal) of long-term use for glaucoma have varied. They include both dramatic IOP reduction with good functional status on one hand, and progression to blindness on the other.
Summarizing the published literature, the National Academies of Sciences, Engineering, and Medicine (NASEM) concluded this year that “there is limited evidence that cannabis or cannabinoids are ineffective for improving IOP associated with glaucoma.”
Physicians should educate patients wanting to try cannabis for glaucoma about its limitations, Dr. Radhakrishnan said.
Side effects may include addiction, structural brain changes, decreased intelligence quotient (IQ), impaired cognitive function, development of psychotic disorders, chronic obstructive pulmonary disease, and increased risk of motor vehicle accidents.
Patients may already be using marijuana recreationally or for medical reasons other than glaucoma. It is important to advise that marijuana does not replace ongoing glaucoma therapy and that regular monitoring for glaucoma for should still be continued.