Abstract INTRODUCTION: Cyclosporine is a molecule used in ophthalmology for the prevention of corneal graft rejection. The systemic use of this product can lead to serious adverse side effects that can be avoided using the topical formulation of cyclosporine. However, cyclosporine application can induce ocular irritation. MATERIAL AND METHODS: The aim of this study is to evaluate the cytotoxicity of four formulations of 2% cyclosporine eye drops: Sandimmum intravenous solution diluted with NaCl 0.9%, Sandimmun oral solution diluted in castor oil or corn oil after ethanol evaporation, and Sandimmun oral solution diluted in castor oil without previous ethanol evaporation. Two tests--the Draize test and the evaluation of cytotoxicity of adherent alive cells with cold light cytofluorimetry on microplates--were used in this study. RESULTS: These tests demonstrated that the aqueous solution shows more toxicity than the other formulations, and the type of oil and ethanol concentration influence cell viability. CONCLUSION: These results helped the Pharmacy unit choose the vehicles for a safe cyclosporine eye drop formulation and thus decrease the side effects of cyclosporine eye drop instillation with a decrease in ethanol concentration compared to published formulations.