I read that there are side effects of using peppermint oil capsules, as with any drug, but are they severe?

Generally Safe: Probably safe in the short term (less than 8 weeks). Heartburn, allergic reactions, flushing, headache and mouth sores are most common. Since peppermint is processed in the liver, check with your physician to make sure that there are no interactions with other medications you are taking that are also processed along similar pathways in the liver.

Peppermint oil: Ingestion of too much peppermint oil can cause nausea/ vomiting, abdominal pain, v heart rate, respiratory problems, dizziness, seizures or loss of consciousness.