Drug interactions are frequent and dangerous, especially for elderly people, for several reasons - elderly people are often treated wtih larger number of medicines at the same time, there are physiological changes that affect the pharmacokinetics of a number of medicines, and they are also more sensitive to the effects of some medicines - so there are changes also at the level of pharmacodynamics. Dietary supplements are also more often used by seniors than by other age groups. This article provides an overview of drug interactions and drug and dietary drug interactions with a focus on the elderly. The introduction summarizes the typology of interactions and then the general principles of interactions in the individual pharmacokinetic phases with examples. The next part is then devoted to interactions dietary supplements and drugs at the level of pharmacokinetics.