L-tyrosine, also known as tyrosine or 4-hydroxyphenylalanine, is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid, meaning that the body can produce it on its own from another amino acid called phenylalanine. L-tyrosine is more hydrophilic than phenylalanine and is encoded by the codons UAC and UAU in messenger RNA.

L-tyrosine is used in the body to produce several important brain chemicals called catecholamines, including dopamine and norepinephrine. It is also used to make enzymes, thyroid hormones, and melanin. L-tyrosine is found in dairy products, meats, fish, eggs, nuts, beans, oats, and wheat.

L-tyrosine supplements are recognized as generally safe by the U.S. Food and Drug Administration (FDA), but they may cause interactions with certain medications like thyroid hormone and monoamine oxidase inhibitors. L-tyrosine supplementation may help alleviate acute stress-induced cognitive decline by restoring catecholamine levels in the brain. It has also been studied for its potential to improve mental performance in stressful situations and to help with depression, but more research is needed to fully understand its role in the body.