Which statement describes an anticoagulant interaction with antipsychotics?

• A. Decreased INR with chlorpromazine or haloperidol
• B. Increased INR with chlorpromazine only
• C. No interaction affecting INR
• D. INR decreases with all antipsychotics

Answer: (A)

When an anticoagulant like warfarin is used with antipsychotics, drug interactions can change how strongly the blood is thinned, as reflected by the INR. Antipsychotics can influence warfarin’s effects by altering liver enzyme activity that metabolizes warfarin, by changing how warfarin binds to proteins, or by affecting other pathways that influence vitamin K and clotting factors. This means the INR can go up or down depending on the specific drugs and the patient.

The statement describing a decreased INR with chlorpromazine or haloperidol captures a possible direction of interaction observed in some cases. It acknowledges that coadministration does not always lead to bleeding risk from an increased INR; in some instances the anticoagulant effect can be reduced, which is clinically important to monitor. This makes it a more accurate description than saying there’s no interaction or that all antipsychotics always decrease the INR, and it’s more specific than claiming an increased INR with chlorpromazine alone, since the interaction direction can vary by drug and context.

In practice, starting or stopping an antipsychotic in a patient on warfarin warrants checking the INR within a few days to a couple of weeks and adjusting warfarin dosing as needed, while watching for signs of bleeding or thrombosis.