NAPQI (N-acetyl-p-benzoquinone imine) is a toxic metabolite of acetaminophen (paracetamol) produced in the liver during its metabolism. It matters in clinical practice because excessive accumulation of NAPQI, particularly in cases of acetaminophen overdose, can lead to hepatotoxicity and liver failure. Clinicians must be vigilant for signs of overdose and consider N-acetylcysteine as an antidote to replenish glutathione levels and mitigate NAPQI's harmful effects.