Paul Palmberg, MD, PhD, a glaucoma expert and National Institute of Health (NIH) medical marijuana panelist, said at a Feb. 20, 1997 conference:
I don't think there's any doubt about its [marijuana] effectiveness, at least in some people with glaucoma.

GW Pharmaceuticals stated on its website (Jan. 2004):

The ability of cannabis and THC to lower intra-ocular pressure in glaucoma was serendipitously discovered in the late 1970's
by a variety of patients and researchers. Several patients in the US Compassionate Use Investigational New Drug Program
maintained their vision while employing large amounts of daily cannabis in situations where standard drug therapy failed....

An emerging concept is that glaucoma represents a progressive vascular retinopathy that requires a neuroprotectant to preserve vision.
Some of the resulting optic nerve damage accrues due to NMDA hyperexcitability, an effect that THC and CBD may counter as neuroprotective antioxidants.

Thus, glaucoma is an area where cannabis and cannabinoids may offer particular advantages over available single ingredient ocular anti-hypertensive agents. Delivery methods remain an exacting challenge.

Jan. 2004 - GW Pharmaceuticals