Currently the most widely prescribed long term anticoagulant drugs are anti vitamin K agents (AVK). They should be used with caution because they are drugs that have a narrow therapeutic range: the difference between the effective dose and toxic dose is small. Their use thus requires careful laboratory monitoring. They can cause bleeding complications in the case of overdose. The AVKs have two antidotes: Vitamin K and prothrombin complex concentrates (PCC). These reverse the anticoagulant effect.