Warfarin is a blood thinner with a narrow safety margin, and many medicines change how it works. NSAIDs such as ibuprofen, naproxen, and aspirin increase the risk of bleeding both by their own effect on the stomach and platelets and by potentially affecting warfarin levels, so they are usually avoided.
Paracetamol is the safer first choice for pain or fever, but even regular high-dose paracetamol can raise the warfarin effect over several days, so tell your doctor or anticoagulation clinic if you start taking it frequently, as your INR may need checking.
Always clear any new medicine, including over-the-counter products and herbal supplements, with the clinician managing your warfarin. Seek urgent care for unusual bruising, nosebleeds that will not stop, blood in urine or stool, or black tarry stools.