Possible side effects of menthol, or peppermint, include heartburn and symptoms associated with an allergic reaction, which include mouth sores, headache and flushing, according to WebMD. Peppermint and peppermint oil are likely safe when used topically or when taken orally.
Women who are nursing or pregnant may want to speak to a physician before taking menthol in medicinal quantities, notes WebMD. People who have a condition called achlorhydria should not use enteric-coated peppermint oil. Enteric-coated peppermint oil can also cause people who have diarrhea to experience anal burning. It is unknown if using peppermint leaf long-term is safe, as of 2015. Menthol may interact with some medications.