Creosote

Background:

Creosote bush resin was sometimes used in herbal remedies, and some bartenders have spotted it being used in tinctures or garnishes for its aroma. 

Legal Limitations:

In the United States, "beechwood creosote" is listed as a "Synthetic flavoring substances and adjuvants" [link]  as long as "they are used in the minimum quantity required to produce their intended effect, and otherwise in accordance with all the principles of good manufacturing practice." 

Safety Information

The United States Center for Disease Control and Prevention states [link]:

Creosote is a mixture of many chemicals. Eating food or drinking water with high levels of creosote may cause burning in the mouth and throat, and stomach pain. Long-term contact with creosote has been associated with increased risk of contracting cancer.

Eating food or drinking water contaminated with high levels of creosotes may cause a burning in the mouth and throat, and stomach pains. Taking large amounts of herbal remedies containing creosote bush leaves may cause damage to the liver or kidney.

Brief direct contact with large amounts of coal tar creosote may result in a rash or severe irritation of the skin, chemical burns of the surfaces of the eyes, convulsions and mental confusion, kidney or liver problems, unconsciousness, and even death. Longer direct skin contact with low levels of creosote mixtures or their vapors can result in increased light sensitivity, damage to the cornea, and skin damage. Longer exposure to creosote vapors can cause irritation of the respiratory tract.