mint oil
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Peppermint oil has a calming effect, which combined with its mild antibacterial and antiviral powers, makes it useful for treating a variety of health conditions. Research is ongoing, but peppermint oil seems to be beneficial for treating indigestion, headaches and irritable bowel syndrome and shows promise for treating other conditions, including bad breath, congestion and nausea. As with any herbal remedy, peppermint oil has potential side effects that you should discuss with your doctor before making peppermint oil a part of your health care regimen.

Stomach Problems

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Though peppermint is often used to soothe digestive issues, its relaxing effect on stomach muscles may sometimes increase symptoms of heartburn and indigestion, according to the University of Maryland Medical Center. Peppermint can relax the sphincter -- the muscle that connects the stomach to the esophagus -- so much that stomach acid flows back up into your esophagus. People who have gastrointestinal issues like gastroesophageal reflux disease, also known as GERD, or hiatal hernia may be especially prone to this side effect. The National Center for Complementary and Alternative Medicine says that when you take peppermint oil capsules with with antacids, the coating on the peppermint oil capsule can break down more quickly, increasing your risk for heartburn and nausea.

Allergic Reactions

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Some people who take peppermint oil capsules experience allergic reactions, including breathing problems, skin irritation like rashes, hives or inflammation, or throat closing, according to MedLine Plus, the online health information resource maintained by the U.S. National Library of Medicine and the National Institutes of Health. Higher doses of peppermint oil may increase risk for allergic reaction, so MedLine Plus suggests limiting your intake to the 0.2 to 0.4 milliliters of peppermint oil taken three times a day to reduce your risk.

Other Side Effects

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There's a low risk of other side effects from taking peppermint oil capsules, including headaches, dizziness, slow heartbeat and anal burning. Very large quantities of peppermint oil have the potential to cause more serious problems, including seizures, muscle weakness and brain damage. If too much peppermint oil is ingested there is a risk of peppermint oil overdose, according to Medline Plus. Symptoms of overdose include dizziness, slow heart rate, change in breathing, nausea, vomiting and convulsions. If you experience any of these symptoms after ingesting peppermint oil, seek emergency medical care as soon as possible.