A tonographic study of one pilocarpine hydrochloride-physostigmine (eserine) salicylate combination did not disclose any evidence of additive effects. A study of several combinations of miotics with levo-epinephrine hydrochloride revealed truly additive effects, probably due to functional interaction of the drugs involved. The study suggests that epinephrine exerts its resistance-lowering effect by action on essentially the same section of the outflow channels as that influenced by the miotics.
Kronfeld PC. The Efficacy of Combinations of Ocular Hypotensive DrugsA Tonographic Approach. Arch Ophthalmol. 1967;78(2):140-146. doi:10.1001/archopht.1967.00980030142006