Adverse drug reactions (ADRs) and interactions not only cause hospitalisation but also occur during the hospital stay itself. There, they contribute significantly to patient morbidity and mortality and furthermore create considerable additional costs for the healthcare systems. The majority of ADRs are dose-dependent and commonly found also for long-established and well known drugs. This is inherently related to the drug's mode of action. Accordingly, even rare side effects like rhabdomyolysis or arrhythmia turn out to be the limiting factor in the use of certain drugs which may even result in their withdrawal from the market. Detection and analysis of ADRs directly in the hospitals is therefore highly important. This provides a unique opportunity to investigate serious cases leading to hospitalisation, and, also prevent further occurrence of ADRs after discharge from hospital and thus enhance the protection of the patients. Clinically most relevant interactions have been found for drug metabolizing enzymes (e.g. CYP3A4, 1A2, 2D6), and drug efflux transporters (e.g. P-glycoprotein). Data based drug-interaction software is an important tool for physicians to either reduce ADRs or to select alternatives with lower interaction potential.