L-tyrosine is an amino acid that's often used in supplements to improve cognitive function affected by stress. Supplements containing L-tyrosine supplements may also help treat phenylketonuria (PKU). L-tyrosine is generally recognized as safe (GRAS) by the FDA. However, as a supplement, the safety of high doses and long-term use remains unknown. Some have reported nausea, fatigue, and headaches with L-tyrosine supplementation. L-Tyrosine has very low toxicity. There have been very few reports of toxicity. LD50 (oral, rat) > 5110 mg/kg.