Absinthe has many impurities! Bad for you to drink!

Fusel oil contaminants:

A mixture of organic acids, higher alcohols (propyl, butyl, and amyl), aldehydes, and esters, known collectively as congeners, produced in alcoholic fermentation.

It is present in low concentration in wines and beer, and in high concentration in pot-still spirit. On maturation of the liquor fusel oil changes and imparts the special flavour to the spirit. Many of the symptoms of hangover can be attributed to fusel oil in alcoholic beverages. Can cause acute illness, including headaches, nausea, vomiting, clinical depression, or coma.

Columbia Encyclopedia: fusel oil

(fyū'zəl) , oily, colorless liquid with a disagreeable odor and taste. It is a mixture of alcohols (largely amyl alcohols) and fatty acids, formed during the alcoholic fermentation of organic materials. After imperfect distillation of these fermentation products it becomes an impurity in the distilled liquor. Fusel oil is used as a solvent in the manufacture of certain lacquers and enamels (it dissolves nitrocellulose). It has a detrimental effect on the human system.