How a Flower Can Kill a Rattlesnake: The Medical Dangers of Pennyroyal Oil

While doing some historical medical research from the 1600s, I recently came across a fascinating account of how people in Virginia used a common flower to kill rattlesnakes. The story was relayed by Captain Silas Taylor to members of the Royal Society in England, who were always interested in hearing new discoveries from places overseas.

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Captain Taylor spoke about how colonists in Virginia used a plant known as pennyroyal (pictured below) to kill rattle snakes. He noted that the leaves of this plant produced a very hot sensation when placed on the tongue.  So the colonists took some of the pennyroyal leaves, tied them to the end of a long stick, and held them by the nose of the rattlesnake.

When exposed to the pennyroyal leaves, the rattlesnakes would turn and wiggle and do whatever they could to avoid it. But the colonists were persistent and eventually the rattlesnake died in less than 30 minutes from the scent of these leaves.

How is this possible? Medical science has the answer. As it turns out, the essential oil in pennyroyal is very high (up to 90%) in pulegone. This oil is highly toxic, particularly to the liver, even at very low levels. (e.g., one ounce). The rest of the oil is made up of similar toxins. Human consumption of just a half a teaspoon of the oil can result in death.

The high toxicity of pulegone is mainly due to methofuran, an organic chemical that the body converts the oil into. Pennyroyal oil can cause seizures, fainting, failure of multiple organs, acute (sudden) kidney and/or liver failure, brain damage, hallucinations, paralysis of respiratory muscles, failure of the heart and lungs, coma, and as mentioned, death. In humans, the toxicity usually occurs a few hours after ingestion, but if one held the flower over the rattlesnake for 30 minutes, the constant exposure to the oils in the flower could explain why death occurred during that time frame.

Pennyroyal oil causes damage to organs by depleting levels of glutathione, a natural chemical in the body that prevents damage to cells. Thus, when not enough glutathione is present, cellular damage occurs quickly.
Humans need to make sure not to ingest pennyroyal oil. For example, in 1996, two infants died because they drank a tea that was made with pennyroyal.  Dogs have died after licking this oil off of their fur. A college student who drank two teaspoons of pennyroyal oil in teas died two days later in 1994. Thousands of years ago, pennyroyal oil was actually used to terminate unwanted pregnancies. It is also used as a powerful insect repellent.

Reference: 1665 (author unknown). Of A Way of Killing Rattle-Snakes. Philosophical Transactions, 3, 43.