The most well-known toxic substance with a licorice smell is 4-methylcyclohexanemethanol, commonly called MCHM, an industrial chemical used in coal processing. Its trans isomer produces a distinctive licorice-like odor that humans can detect at extraordinarily low concentrations. But MCHM isn’t the only dangerous substance with this scent profile. Several other toxic compounds carry sweet, anise-like notes that overlap with what most people recognize as licorice.
MCHM: The Industrial Chemical With a Licorice Scent
MCHM is a colorless liquid alcohol used as a foaming agent in coal washing. It exists in two molecular forms, called cis and trans isomers. The trans form is the one responsible for the recognizable licorice smell, while the cis form is far harder to detect by nose. What makes MCHM particularly noteworthy is how little it takes to smell it: the human nose can pick up the trans form in air at concentrations as low as 0.060 parts per billion. People who have been previously exposed to MCHM become even more sensitive, detecting it at roughly one-third of that level.
MCHM gained national attention in January 2014 when roughly 10,000 gallons of it leaked from a storage facility into the Elk River in West Virginia, contaminating the drinking water supply for over 300,000 people in the Charleston metropolitan area. Residents reported a strong licorice odor coming from their tap water. In the water itself, people could smell crude MCHM at concentrations around 0.55 parts per billion, and more than 35% of individuals could detect it during showering even at 2 parts per billion, which was the lowest level analytical equipment could reliably measure at the time.
Health Effects of MCHM Exposure
MCHM causes irritation to the skin, eyes, and respiratory tract. During the West Virginia spill, 369 people visited emergency departments, and nearly 2,000 called the state poison center. The most common symptoms among emergency patients were nausea (38%), skin rash (28%), vomiting (28%), abdominal pain (24%), and diarrhea (24%). Headache affected 22% of patients, while eye pain, sore throat, and cough were each reported by 13 to 15%.
People were exposed through three main routes: direct skin or mucous membrane contact (53% of emergency patients), ingestion through contaminated water (44%), and inhalation (15%). Many reported more than one route. The CDC set a short-term safe drinking water level for MCHM at 1 part per million (1,000 parts per billion), stating that concentrations at or below this threshold were unlikely to cause adverse health effects. For pregnant women, the CDC recommended water below 50 parts per billion, which is one-twentieth of that screening level.
Anethole: The Compound Behind Licorice Flavor
Anethole is the organic compound that gives licorice, anise, and fennel their characteristic sweet taste and smell. In small amounts found naturally in food, it’s generally harmless. In concentrated form, however, anethole is genuinely toxic. Applied to skin in pure form, it causes redness, scaling, and blistering. In animal studies, one milliliter applied to mouse skin produced coma within half an hour and death within two and a half hours, with significant kidney damage found on examination.
High-dose feeding studies in rats caused liver inflammation, drowsiness, and coma. Fennel oil, which is 50 to 80% anethole, can cause vomiting, seizures, and fluid buildup in the lungs if swallowed in toxic amounts. People who handle anethole-containing products occupationally can develop contact dermatitis, and at least one case of mouth inflammation has been linked to a denture cream containing anise oil (which is 80 to 90% anethole). If you encounter an unfamiliar liquid with a strong licorice smell, anethole or an anethole-containing essential oil is one possibility to consider.
Pennyroyal Oil and Liver Damage
Pennyroyal oil comes from plants in the mint family and has a strong fragrance often compared to spearmint, though some people perceive a sweet, licorice-adjacent quality. Its primary toxic component is a compound called pulegone, which makes up about 80 to 85% of the oil. The liver converts pulegone into a highly toxic byproduct that destroys liver cells.
Pennyroyal was historically used to flavor food, wine, and herbal teas, and it has a long history of misuse as a folk remedy. Poisoning cases typically develop within a few hours of ingestion, producing acute liver damage that can progress to full liver failure and death in severe cases. Because of the overlap in sweet, herbal scent notes, pennyroyal oil is worth knowing about for anyone trying to identify an unknown substance by smell.
Other Substances With Licorice-Like Odors
- Glycyrrhizin is the naturally sweet compound in licorice root. It isn’t a classic “poison,” but consuming large amounts (through supplements or excessive licorice candy) can cause dangerously low potassium levels, high blood pressure, and heart rhythm problems.
- Star anise oil smells strongly of licorice due to its high anethole content. Japanese star anise, a related but different species, contains additional toxic compounds that cause seizures and has been involved in poisoning cases when confused with the culinary variety.
- Estragole, found in basil and tarragon oils, has a licorice-like aroma and is structurally similar to anethole. In concentrated form, it has shown liver-toxic and potentially cancer-causing effects in animal studies.
What to Do if You Smell Licorice Unexpectedly
A licorice smell where you don’t expect one, especially in tap water, near industrial sites, or from an unidentified liquid, is worth taking seriously. During the 2014 West Virginia spill, the licorice odor was the first sign that something was wrong with the water supply, detectable by nose long before standard lab equipment could confirm contamination. Your nose is remarkably sensitive to these compounds.
If you notice a strong, unexplained licorice scent in your water, avoid drinking, cooking with, or bathing in it until the source is identified. Ventilate the area if the smell is in the air. Contact your local water utility or environmental health department to report it. For skin or eye contact with an unknown substance that smells like licorice, rinse thoroughly with clean water and contact poison control (1-800-222-1222 in the U.S.) for guidance specific to your situation.