During his last two years Vincent van Gogh experienced fits with hallucinations that have been attributed to a congenital psychosis. But the artist admitted to episodes of heavy drinking that were amply confirmed by colleagues and there is good evidence to indicate that addiction to absinthe exacerbated his illness. Absinthe was distilled from an alcoholic steep of herbs. Wormwood (Artemisia absinthium) was the most significant constituent because it contributed thujone. This terpene can cause excitation, convulsions that mimic epilepsy, and even permanent brain damage. Statements in van Gogh's letters and from his friends indicate that he had an affinity for substances with a chemical connection to thujone; the documented examples are camphor and pinene. Perhaps he developed an abnormal craving for terpenes, a sort of pica, that would explain his attempts to eat paints and so on, which were previously regarded as unrelated absurdities.
Arnold WN. Vincent van Gogh and the Thujone Connection. JAMA. 1988;260(20):3042-3044. doi:10.1001/jama.1988.03410200098033