► UV radiation is effective for the photooxidation of TCs in aqueous phase. ► The rate constants obtained indicate that the photodegradation is pH-dependent. ► The presence of H2O2 in the medium substantially increased the TC degradation rate. ► The TC photooxidation rate was higher in natural waters. ► The medium toxicity increased at the beginning of TC photooxidation process. The objective of this study was to analyze the effectiveness of medium-pressure lamps in the direct and indirect photodegradation of tetracyclines (TCs). We conducted a kinetic study of the process, determining the quantum yield for three tetracyclines (tetracycline, chlorotetracycline and oxytetracycline). We also analyzed the influence of different operational variables, such as: initial concentration; solution pH; presence of natural organic matter (NOM); the chemical composition of water, using ultrapure water (UW), surface water (SW), groundwater (GW), and wastewater (WW); the behavior of total organic carbon (TOC) concentration, and the system toxicity during TCs photodegradation. The results obtained shown that medium-pressure mercury lamp radiation is effective for the photooxidation of TCs in aqueous phase, mainly when these are at low concentrations. The rate constants obtained indicate that the photodegradation is pH-dependent and favored at pH 10 regardless of the tetracycline considered. The presence of H2O2 in the medium substantially increased the TC degradation rate and considerably reduced the time required to degrade 100% of the initial TC concentration. The TC photooxidation rate was higher in natural waters than in ultrapure water, attributable to the presence of dissolved organic matter, which can act as a photosensitizer, thereby favoring indirect TC degradation through the formation of radical species such as HO•. TOC values at the end of the treatment indicated that the percentage mineralization is a function of the initial TC concentration. The medium toxicity increased at the beginning of TC photooxidation, and treatment times of around 60min are generally required for the toxicity to be lower than in the original solution.