The liver has many physiological roles and is a key organ in detoxication. Furthermore, the liver is affected by a very large number of xenobiotics. The liver also has an immense capacity for self‐repair, so that the many abnormalities of structure or function in the liver reverse rapidly on cessation of xenobiotic exposure. Nevertheless the liver's defences can be overwhelmed. Acute liver failure can be caused by many compounds e.g. paracetamol (acetaminophen), while repeated liver damage, for example from ethanol, may lead to cirrhosis. While primary liver cancer is rare in Western Europe and North America, it is common in other parts of the world such as sub‐Saharan Africa, alcohol and aflatoxins being amongst the aetiological factors. Non‐dose‐dependent adverse effects on the liver are common with many classes of drug and may be sufficiently severe to require cessation of treatment. In the regulatory context it is important to recognize the difference between adaptive and toxic effects.