Can marijuana be used to treat glaucoma?

Yes. It can be, but the effect is very short-lived and currently not practical as an effective widespread therapy for glaucoma, especially because of the side effects. Research has been done on the active ingredient, but so far it is not more effective than the meds already on the market.
Yes. Glaucoma is one of the first conditions for which there is medical evidence that medicinal marijuana can be helpful. However, there are different types of glaucoma. You should consult your ophthalmologist prior to beginning this therapy.