A patient presented to our clinic with stage Ia open-angle glaucoma of the right eye and stage IIa surgically corrected open-angle glaucoma of the left eye. The condition of the ocular surface was interpreted as toxic/allergic conjunctivitis provoked by brimonidine 0.15 %. Brimonidine was substituted with non-selective 0.5%; additionally, topical steroids were prescribed. After steroids were discontinued, some of the symptoms came back, including moderate hyperemia and conjunctival edema, which was interpreted as intolerance to a preservative contained in the eye drops. A decision was made to switch from the β-blocker to its preservative-free formulation; regular IOP monitoring was continued. IOP measured during the next visit was above tolerated, so a preservative-free form of the ocular hypotensive combination drug (an analog of prostaglandin 0.005% with non-selective β-blocker 0.5%) was introduced to the regimen, with further IOP monitoring. Because the initial diagnosis was wrong, damage to the ocular surface had been aggravated by inadequate therapy. Preservative-free hypotensive eye drops are beneficial for the corneal surface and have a positive effect on a patient’s adherence to the regimen.