Harvard Women's Health Watch

By the way, doctor: What can you tell me about peppermint oil?

Q. What are the pros and cons of taking peppermint oil?

A. Peppermint oil is extracted from parts of the peppermint plant, an herb that's been used as a digestive aid since ancient times. Nowadays, it's a flavoring agent in many over-the-counter health products, including toothpastes and mouthwashes. Menthol, a component of peppermint, is an ingredient in topical preparations for conditions ranging from congestion to muscle aches.

Peppermint oil relaxes the smooth muscle cells that line much of the gastrointestinal tract. It has been most extensively studied as a treatment for irritable bowel syndrome (IBS) and indigestion. Although clinical studies in people with IBS have shown mixed results, two major analyses found a modest benefit. People with IBS who took peppermint oil reported less flatulence, abdominal pain, and bloating compared with those who took a placebo. Given by enema, peppermint oil can help relieve intestinal spasms in people under going a barium examination of the large intestine.

Unfortunately, peppermint also relaxes the sphincter between the esophagus and stomach (the lower esophageal sphincter), so it can cause gastroesophageal reflux and heartburn. It can worsen reflux symptoms in people with hiatal hernia and — not surprisingly — those with gastro esophageal reflux disease (GERD). In studies of people with IBS, peppermint oil (generally 0.2 to 0.4 milliliters three times a day) is taken in enteric-coated capsules, which allow it to bypass the esophagus and stomach before it is broken down and metabolized.

Peppermint oil has other downsides besides its effects on the lower esophageal sphincter. At high doses, it can be toxic to the kidneys, and you should not take it if you have gallstones or active gallbladder inflammation. Finally, check with your doctor if you take any other medication, because peppermint oil can boost the blood level of some medications, including the antidepressant amitriptyline (Elavil, others) and the statin simvastatin (Zocor).

— Celeste Robb-Nicholson, M.D.
Editor in Chief, Harvard Women's Health Watch