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Apple cider vinegar is highly acidic. The main ingredient of apple cider vinegar is acetic acid. As the name suggests, it's quite harsh. Apple cider vinegar should always be diluted with water or juice before swallowed.
Pure apple cider vinegar could damage the tooth enamel and the tissues in your throat and mouth. One study found a woman who got an apple cider vinegar supplement stuck in her throat. She seemed to have suffered lasting damage to her esophagus. Vinegar has been known to cause contact burns to the skin.
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Long-term use of apple cider vinegar could cause low potassium levels and lower bone density. If you already have low potassium or osteoporosis, talk to your doctor before using apple cider vinegar.
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Apple cider vinegar could theoretically interact with diuretics, laxatives, and medicines for diabetes and heart disease. *
If you have diabetes, check with your doctor before using apple cider vinegar. Vinegar contains chromium, which can alter your insulin levels.
http://www.webmd.com/diet/apple-cider-vinegar?page=2
