Light-related adverse side-effects of drugs are now an important source of concern. In order that the mechanism underlying to such effects is recognised, an in-depth photochemical study must be carried out. The case of some fluoroquinolone antibiotics (norfloxacin, enoxacin, lomefloxacin) is discussed as a representative example. These drugs undergo heterolytic fragmentation of the C—F bond leading to aryl cations. Quantum yields in neutral water range from 0.001 to 0.5 depending on the charge transfer degree of the ππ* state. Lower values are obtained at both acidic and basic pH. There is indication that the excited state involved in most of this reactions is a relatively long-lived triplet. The aryl cations undergo intra- or intermolecular reactions, and are presumably involved in the reported photo-toxic effect.