Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trial of a drug cannot be directly compared to rates in clinical trials of another drug and may not reflect the rates in practice. In seven clinical trials, patients with allergic conjunctivitis or those at a risk of developing allergic conjunctivitis received one drop of either cetirizine (N=511) or vehicle (N=329) in one or both eyes. The most commonly reported adverse reactions occurred in approximately 1–7% of patients treated with either ZERVIATE ® or vehicle. These reactions were ocular hyperemia, instillation site pain, and visual acuity reduced. The most common adverse reactions (1–7%) were ocular hyperemia, instillation site pain, and visual acuity reduced.