Which of the following over-the-counter medications has a high risk of causing a serious drug-to-drug interaction with warfarin (Coumadin)?

Diphenhydramine is an antihistamine primarily used for allergy relief and does not have a significant interaction with warfarin. While it may cause sedation or other side effects, it does not enhance the anticoagulant effects of warfarin and is generally considered safe for use in patients on this medication.

Famotidine is a histamine-2 blocker used to treat conditions like heartburn and gastric ulcers. It does not significantly interact with warfarin and is often safely prescribed to patients taking anticoagulants. Its mechanism of action does not influence warfarin's effectiveness or increase bleeding risk.

Acetaminophen is a common pain reliever and fever reducer that is generally considered safe for use with warfarin at recommended doses. However, excessive use can potentially lead to liver damage, which may indirectly affect warfarin metabolism, but it does not pose a direct serious interaction like aspirin does.