What is a major adverse effect associated with anticoagulant drugs?

Bleeding is a major adverse effect associated with anticoagulant drugs due to their mechanism of action, which involves inhibiting the blood coagulation process. Anticoagulants, such as warfarin and direct oral anticoagulants, are designed to decrease the blood's ability to clot, thereby preventing thrombosis and embolism. However, this reduction in clotting capability leads to an increased risk of bleeding complications, which can manifest as easy bruising, prolonged bleeding from cuts, and potentially severe internal bleeding.

The other options listed, while they can be side effects of various medications, are not specific concerns associated with anticoagulants. Constipation is more commonly related to certain pain medications or iron supplements, headache can arise from numerous causes unrelated to anticoagulation, and fever is typically indicative of infection or inflammation rather than a direct effect of anticoagulant therapy. Thus, understanding the inherent risks associated with anticoagulants emphasizes the necessity for careful monitoring of patients receiving these medications to manage bleeding risks effectively.