Can eye drops for glaucoma stop you going blind or does it just slow the deterioration?

Yes: Both. Eye drops slow the rate of deterioration by lowering the pressure, which stops one from going blind. Thus the key factor is testing the eye pressure and fields for the rate of change at the given pressure range (minus subjective variability).

Both: Most eye drops for glaucoma lower the eye's pressure. The lower the pressure, the slower the damage to the vision. The goal is to slow the damage to as close to a standstill as possible. The amount of pressure lowering needed to stop the vision loss is different for everyone and the eyes must be monitored closely by an eye doctor to ensure that no further vision loss is occurring.

Both: Glaucoma drops lower eye pressure and slows progression of vision loss. This can prevent blindness in many cases. However if you already have vision loss from glaucoma , it is irreversible and drops only help prevent further loss. Often surgery is required when the damage is severe