Drug interactions are commonly present in our daily clinical work and have the ability to either enhance or reduce the effects of a drug, but also, just as importantly, decrease or increase the risk of side effects, thereby increasing the risk of toxicity. This chapter will focus on the clinically important drug–drug interactions with some of the most often used cardiovascular drugs and especially where there is a risk of causing harm to patients due to serious side effects. Both pharmacokinetic and pharmacodynamic interactions will be discussed, with focus on the most vulnerable patients. In some cases, drug combinations are contraindicated, whereas in other cases, the combinations can be used with, for example, close monitoring or dose reductions, but most importantly, the risk of interactions should be considered in order to take appropriate precautions upfront.