Drug interactions are defined as quantitative or qualitative changes in the drug effect caused by the administration of a second drug or additional substances. A basic distinction is made between pharmacokinetic (leading to changes in the processes a substance undergoes in the body - e.g. absorption and excretion) and pharmacodynamic interactions. In pharmacodynamic interactions, the processes lead to changes in the substance's effect on the organism. In particular, interactions must be taken into account when a substance has a narrow therapeutic range. This also applies to the use of a pharmacon in elderly and multimorbid patients. Even in the case of rational phytopharmaceuticals, interactions must be expected due to the usually multiple "co-determining" ingredients.