L-Tyrosine is an amino acid precursor of the neurotransmitter, norepinephrine (a derivative of dopamine).
      Elevated levels of the neurotransmitters serotonin, dopamine and acetylcholine definitely enhance immune function. Norepinephrine has the greatest effect, increasing T cell response, macrophages, and stem cells in bone marrow.
      L-Tyrosine increases levels of norepinephrine.

      Combining L-Tyrosine with carbohydrates and vitamin C enhances its ability to induce norepinephrine production.

      Additionally, studies performed by the United States Army Research Institute of Environmental Medicine (Natick, Mass.) showed that while control and experimental groups of soldiers taking L-Tyrosine demonstrated no marked differences under non-stressful conditions, the benefits of tyrosine become manifest as physiological and psychological conditions become harsher.
          Under progressively harsher conditions, L- Tyrosine soldiers showed enhancements in performing complex mental tasks and decision-making, alertness and response times, mood elevation (less tense or anxious), and resistance to physical discomfort.

      Dr. Jacques Mouret reported in a 2/89 issue of Lancet that L-Tyrosine can free narcolepsy patients from the irresistible desire to sleep.
      Narcolepsy affects approx. 250,000 Americans and about four times as many men as women. Narcolepsy also causes victims to fall limp from a loss of muscle tone triggered by laughter, anger or other bursts or emotion (the loss of tone is called cataplexy).

      Eric Braverman, in the 01/01/90 issue of Longevity, notes that up to 85 percent of people diagnosed as having chronic fatigue syndrome actually suffer a biochemical depression treatable with L-Tyrosine or phenylalanine.

      Psychologist Peter Carlton of the Robert Wood Johnston Medical School in Piscataway, NJ, has indicated low levels of the neurotransmitter serotonin in compulsive gamblers.
      EEG reading and other evidence of the gambler's brain activity parallels those of children with attention deficit disorder. Children who suffer from hyperactivity may have lowered levels of serotonin, which has inhibitory effects.

      There is a link between depression, violent suicide, hyperactivity in children, compulsive gambling behavior, and low levels of both of the neurotransmitters, serotonin and norepinephrine. As previously mentioned, L-Tyrosine increases levels of norepinephrine.

      While toxicity resulting from the use of L-Tyrosine is practically nonexistent, it can cause sweats and mild blood pressure elevation if taken in conjunction with monoamine oxidase inhibitors (MAOI).