It can cause irritation when applied to the skin. Because peppermint may increase symptoms of acid reflux disease and hiatal hernia, people with these conditions are advised to avoid the herb. People with gall stones or liver damage should also use caution when using peppermint. Peppermint oil and menthol products should not be applied to the nose or face of an infant or small child since they could cause trouble breathing.
Rare cases of allergic rashes, hives, asthma, and other reactions have been reported in people who are sensitive to peppermint or its components.
Acid-reducing medicines may allow enteric-coated peppermint to be released before it leaves the stomach, which may reduce its effect on an irritable bowel. Peppermint oil may interact with other medicines as well, so it is helpful to talk with your doctor or pharmacist about all medicines and supplements that you are taking.
Menthone (which makes up around 20% of peppermint oil) was given to rats for 4 weeks by mouth. It affected kidney and liver function, and very high doses caused cyst-like spaces in the brainstem. In other animal studies, large doses have caused weakness, seizures, and brain damage.
Menthol is considered to be the most toxic component of peppermint oil. As little as 2 grams are thought to be fatal in some, although people have survived higher doses.
In work settings, those who handle menthol are cautioned that it can irritate the eyes. Eyes should be thoroughly flushed if contact occurs. For skin contact, wash with soap and water. Stomach upset with pain, vomiting, vertigo, drowsiness, and coma may result from ingestion, and death may occur due to lung failure.
Peppermint oil should not be taken by injection; a recent report describes the case of a young woman who had serious lung damage after intravenous injection of peppermint oil. She survived but required intensive care and the support of a breathing machine for 13 days. Taking any type of oil by injection can cause serious effects, including death.
Peppermint oils, when taken by mouth, can affect the way that other drugs are absorbed by the body and may interfere with antacids, medicines for high blood pressure, and others. Mint salves and creams can cause the ingredients in other creams and ointments to be absorbed more quickly. Other potential interactions between peppermint and other drugs and herbs should be considered. Some of these combinations may be dangerous. Always tell your doctor and pharmacist about any herbs you are taking. Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.