The Side Effects of Lysine to Reduce Cold Sores

Lysine is an essential amino acid required to support growth and development, wound healing and the metabolism of fats into energy. Lysine is also famous, according to the University of Pittsburgh Medical Center, as a treatment for cold sores 1. For the latter, UPMC recommends lysine supplements in doses up to 3 g per day. Other experts, such as the University of Maryland Medical Center, recommend even higher doses — up to 9 g per day during an active outbreak 1. Although lysine supplements are generally safe, as with any other dietary supplement, they can occasionally cause side effects.

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If you are experiencing serious medical symptoms, seek emergency treatment immediately.


UPMC says that lysine supplements may cause gallstones, at least in hamsters. As of November 2010, the National Library of Medicine does not list any cases linking lysine supplements to gallstones in humans. However, if you have a history of gallstone disease, it’s a good idea to talk to your physician before starting lysine supplements for cold sores or any other condition. Although many people with gallstones experience no symptoms at all, others experience sharp, cramping or dull pain on the middle or right side of the abdomen, usually within minutes of eating. Other signs and symptoms of gallstones include yellowing of the skin and the whites of the eyes, clay-colored stools, nausea and vomiting after meals and abdominal fullness. If you experience gallstone symptoms, you should see a doctor, even if you are not using lysine supplements.

Heart and Blood Vessel Disease

Lysine potentially exerts both direct and indirect effects on heart and blood vessel disease. A 1982 study published in the journal “Experientia” found that lysine supplementation in chickens increased blood cholesterol. Elevated blood cholesterol, in turn, may result in hardening of the arteries. Lysine also competes with another amino acid, called arginine, for absorption. Since arginine helps dilate blood vessels, taking lysine supplements may indirectly promote blood vessel constriction by decreasing the availability of arginine. As of November 2010, the National Library of Medicine does not list any cases linking lysine supplements to heart attacks or other blood vessel problems. However, many people who use dietary supplements do not share this information with their physicians. If you have a history of heart or blood vessel problems, you should check with your physician before taking lysine or any other dietary supplement.

Gastrointestinal Upset

A drug monograph published in the June 2007 issue of “Alternative Medicine Reviews” lists nausea, abdominal cramps and diarrhea as side effects of lysine doses greater than 10 g per day 2. However, tolerance varies from person to person and some people may experience gastrointestinal upset at doses lower than 10 g per day. Gastrointestinal symptoms occur when unabsorbed lysine sits in the gastrointestinal tract because consumption exceeds the body’s capacity for absorption. Dividing the total dose of lysine into three or four small doses taken throughout the day maximizes absorption and minimizes side effects. If this is not enough, you should try decreasing the total dose in increments of 500 mg to 1 g per day until you find a dose that works for you. You should not just suffer through side effects because lysine your body cannot absorb has absolutely no effect on cold sores.