What is the main adverse effect associated with heparin use?

Heparin is an anticoagulant medication commonly used to prevent and treat blood clots. The main adverse effect associated with its use is bleeding. This is due to its mechanism of action, which involves inhibiting the clotting factors in the blood, thereby increasing the risk of hemorrhage. Since heparin affects the blood's ability to clot, patients receiving this medication must be closely monitored for signs of bleeding, such as unusual bruising, blood in urine or stool, and prolonged bleeding from cuts.

The other options, while they may have some relevance in other contexts, do not represent the primary concern when using heparin. Elevated blood pressure is not a typical side effect linked directly to heparin. Thrombosis is actually what heparin is intended to prevent. Headache can occur as a side effect of many medications but is not a significant or primary adverse effect of heparin specifically. Thus, bleeding is recognized as the most critical risk associated with heparin therapy and requires vigilant monitoring.