Critically ill patients in Intensive Care Units (ICUs) are exposed to multiple procedures and usually require complex treatment regimens. Many of them suffer from comorbidities and different complications such as organ failure, drug-drug interactions, and unusual therapeutic interventions that can produce significant pathophysiologic changes. For that reason, pharmacokinetics for several substances is different to what is described for healthy patients, complicating drug selection and drug dosage to achieve appropriate effects. Low doses may determine a reduction of drug effectiveness and overdoses leading to toxicity. The aim of this paper is to review the pharmacokinetic considerations that must be considered when treating acute ICU patients.