In the elderly, polymedication is frequent. This increases the risk of adverse reactions to medications, and also the risk of interactions that may be relevant. The most frequent interactions are those that affect the pharmacokinetics of drugs and, especially, their metabolism. Here the cytochrome P450 has a lot of relevance, but for a little more than a decade we know another mechanism involved, the membrane transporter proteins. Within these are the OATP (Organic anion transporting polypeptide) of which there are different types and locations. The competition of different substrates for these proteins can generate interactions that end up having an impact on the pharmacotherapeutic treatment of the patient.