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L Cystine Side Effects

By Joe King, M.S. ; Updated August 14, 2017

L cysteine, or just cysteine, is a nonessential amino acid that is required by your body in many different biochemical reactions. Cysteine is marketed in nutritional supplements as an anti-oxidant and immune system booster, and has been reported to improve muscle recovery following vigorous exercise and soft tissue recovery following injury or surgery. Despite its reported benefits, cysteine may cause various side effects. Consult your physician before using any nutritional supplement containing L cysteine.


As a nutritional supplement, L cysteine can be found in a variety of forms, the most common being N-acetyl-L-Cysteine and D-cysteine. The University of Maryland Medical Center states that some forms of cysteine may be toxic to your body and should not be ingested due to health safety concerns. Various forms of cysteine are generally well-tolerated when consumed in doses between 250 mg and 1,500 mg per day. However, high doses of cysteine, upwards of 7 g per day, may be toxic and cause harmful side effects.

Common Side Effects

Most side effects associated with cysteine supplementation are generally mild when taken within the normal range. The most common side effects of cysteine supplementation include upset stomach, nausea, vomiting, diarrhea, persistent stomach cramps, body aches, headache, and general gastrointestinal discomfort. Taking cysteine supplements with adequate amounts of water may reduce the risk of developing gastrointestinal distress and other mild side effects.


Some individuals may experience an allergic reaction to some forms of supplemental cysteine. An allergic reaction occurs when your body responds to cysteine by releasing excessive amounts of homocysteine, a hormone derived from cysteine that is released as a response to toxins in your bloodstream. Symptoms of an allergic reaction to cysteine include the appearance of a rash, swelling of your face and throat, difficulty breathing, and irregular heartbeat. An allergic reaction to cysteine supplements are considered a medical emergency, seek medical attention immediately if you think you are experiencing an allergic reaction.

Drug Interactions

Supplements containing cysteine may have adverse interactions with certain drugs and medications. Cysteine may interfere with angiotensin-converting enzyme, or ACE, inhibitors; prednisione; and cyclophosphamide. Cysteine may block the effects of medications that treat angina, or chest pain, such as nitroglycerin and isosorbide. Consult your physician if you are taking any medications before using a cysteine supplement to make sure there will be no adverse reactions when combining cysteine with other drugs.

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