You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

New avenues for anticoagulation in atrial fibrillation. Vitamin K antagonists (VKAs) were, until recently, the only option for chronic anticoagulation in patients with atrial fibrillation. Three new oral anticoagulants have been tested for this indication in recent randomized trials: dabigatran, rivaroxaban, and apixaban. They offer the promise of consistent effect without the need for monitoring drug levels. Overall, efficacy and safety appear to be noninferior, and in some instances superior, to warfarin. However, the new drugs have their downsides, and switching from warfarin is not recommended for all patients. This review focuses on the available evidence and attempts to provide guidance to clinicians in the field.
{"Chemical": ["Vitamin K", "apixaban", "warfarin", "dabigatran", "rivaroxaban"]}