Phenprocoumon (marketed under the brand names Marcoumar, Marcumar and Falithrom) is a long-acting anticoagulant to be taken by mouth, and a coumarin derivative.

[1] When phenprocoumon therapy is started, the clotting tendency of the blood is measured daily by determining the prothrombin time, more specifically the international normalized ratio (INR).

[1][4] This is necessary because people need different doses depending on the genetic makeup of their enzymes, activity of coagulation factors, vitamin K concentrations in the body, other drugs, and diet.

[2][6] If a fast onset of action is needed, as after an acute thromboembolism, phenprocoumon therapy has to be accompanied with a subcutaneous or intravenous low-molecular-weight heparin (LMWH) for the first 36 to 72 hours.

It occurs in 5–25% of patients and ranges from harmless nosebleeds to life-threatening bleeding in the brain, gut wall, adrenal glands, pleural cavity, pericardium, or subdural space.

Other side effects are uncommon and include headache, nausea, reversible hair loss, purple toe syndrome, and allergic rashes.

[1] Due to its narrow therapeutic index, the fact that it can only be eliminated from the body after inactivation by the liver enzymes CYP2C9 and CYP3A4, and its high plasma protein binding (see below), phenprocoumon has significant interactions with a large number of other drugs and with some kinds of food.

[1][4] The substance is metabolized by the liver enzymes CYP2C9 and CYP3A4 to various hydroxyl derivatives,[12] and subsequently conjugated to glucuronic acid to a small extent.

[4] Polymorphisms of VKORC1, the gene coding for subunit 1 of vitamin K epoxide reductase, can influence the dose needed to achieve the desired INR.

[4] Xabans have a fast onset and cessation of action, wide therapeutic index, and relatively low potential for interactions with other drugs and food.

INR monitoring under phenprocoumon makes it easier to detect problems such as medication errors or interactions with drugs or food.

[3][15][16] LMWHs also have a fast onset of action, wide therapeutic index, standard dosing schemes, and a very low potential for interactions.