Side effects of timolol eye drops for the treatment of glaucoma (a group of eye diseases that cause vision loss through damage to the optic nerve) may include stinging, irritation, blurred vision, tearing, and allergic reaction. Elderly people are especially prone to side effects. It may cause breathing problems for people with asthma. It can slow heart rate for those with heart disease and may cause mental and physical lethargy. Men may experience a decrease in libido.