Heparin

a.k.a. unfractionated heparin is a widely used anticoagulant that is also naturally occurring in mast cells and basophils. Effective at preventing the formation of blood clots and is generally used for acute coronary syndrome, atrial fibrillation, deep-vein thrombosis, and etc. Heparin and its low molecular weight derivatives binds to the enzyme antithrombin (AT) that results in its activation which leads to the inactivation of thrombin and other proteases involved in blood clotting, most notably factor Xa. It is monitored by checking the PTT. A very notable and severe complication of unfractionated heparin is heparin-induced thrombocytopenia (HIT).