General
Description & Use
Used
to prevent blood clots from forming or growing larger. Warfarin
(also known under the brand names Coumadin, Jantoven, Marevan,
and Waran) is an anticoagulant. Warfarin has been found to be
effective and relatively safe for preventing thrombosis and embolism
(abnormal formation and migration of blood clots) in many disorders.
It was approved for use a medication in the early 1950s, and has
remained popular ever since; warfarin is the most widely prescribed
anticoagulant drug in North America. Warfarin is a synthetic derivative
of coumarin, a chemical found naturally in many plants, notably
woodruff (Galium odoratum, Rubiaceae), and at lower levels in
licorice, lavender, and various other species.
Warfarin
and related coumarins decrease blood coagulation by inhibiting
vitamin K epoxide reductase, an enzyme that recycles oxidated
vitamin K to its reduced form after it has participated in the
carboxylation of several blood coagulation proteins, mainly prothrombin
and factor VII. For this reason, drugs in this class are also
referred to as vitamin K antagonists. Warfarin is prescribed to
people with an increased tendency for thrombosis or as secondary
prophylaxis (prevention of further episodes) in those individuals
that have already formed a blood clot (thrombus). Warfarin treatment
can help prevent formation of future blood clots and help reduce
the risk of embolism (migration of a thrombus to a spot where
it blocks blood supply to a vital organ).
Common clinical indications for warfarin use are atrial fibrillation,
the presence of artificial heart valves, deep venous thrombosis,
pulmonary embolism, antiphospholipid syndrome and, occasionally,
after myocardial infarction. Dosing of warfarin is complicated
by the fact that it is known to interact with many commonly-used
medications and even with chemicals that may be present in certain
foods. These interactions may enhance or reduce warfarin's anticoagulation
effect. In order to optimize the therapeutic effect without risking
dangerous side effects such as bleeding, close monitoring of the
degree of anticoagulation is required by blood testing (INR).
During the initial stage of treatment, checking may be required
daily; intervals between tests can be lengthened if the patient
manages stable therapeutic INR levels on an unchanged warfarin
dose. When initiating warfarin therapy ("warfarinization"), the
doctor will decide how strong the anticoagulant therapy needs
to be. The target INR level will vary from case to case depending
on the clinical indicators, but tends to be 2–3 in most conditions.
In particular, target INR may be 2.5–3.5 (or even 3.0–4.5) in
patients with one or more mechanical heart valves.
