In this “Knockout,” our experts compare the pain-relieving and fever-fighting abilities of acetaminophen and ibuprofen. They also compare side effects, toxicity and other risks. For some conditions, one popular over-the-counter drug has the advantage. For others, both medicines work well. But each medication comes with risks, some of them significant. Ask your doctor which medication is best for you and which may prove too risky. For your health, follow dosage directions and make sure your doctor knows how often you’re taking painkillers.