Most of the blame for the reputed deleterious effects of absinthe was directed at the wormwood used to flavor the drink. Wormwood, Angelica absinthium, contains the terpene thujone, which is used for medicinal purposes, but is toxic in high doses. Interestingly, thujone is present in the culinary herbs sage and tarragon and is used to flavor another (non-banned) alcoholic beverage, vermouth. More likely, the hallucinations and purported insanity resulted from the presence of toxic impurities or additives. For example, the green coloration the spirit derives from herbs was sometimes achieved through the addition of cupric acetate and other copper salts. Antimony trichloride was added to some preparations to enhance the louche effect (the opalescent milkiness seen when water is added to the liqueur).

At the time of this writing, authentic absinthe containing thujone is not sold in the United States. The thujone levels in commercially distilled absinthe must be 10 mg/l or less in order to comply with European Union (EU) limits. The thujone levels in commercially distilled absinthe in Germany must be 30 mg/l or less. Herbal kits for making absinthe are widely available, however, these kits tend not to rely on distillation, which is required to make traditional absinthe.