A few natural and many synthetic chemical compounds may cause liver injury, a hepatitis. They are grouped in two classes: toxic and sensitizing agents.

Toxic agents: -affect all individuals.
-the severity is dose related.
-the symptoms appear quickly in a few hours.
-the symptoms appear with the first dose.
-the histological changes are typical for individual toxins.
-can be reproduced in experimental animals.

Sensitizing agents: -affect only sensitive individuals (1 in 100, 1 in 1000s).
-the severity is not dose related.
-the symptoms appear weeks and months after the first sensitizing dose and immediately after the second dose.
-histological pattern similar to that of a viral hepatitis.
-not reproducible in experimental animals.

The mechanisms of action are multiple and will be disussed with individual compounds in the following chapters.