Tyrosine is a precursor to neurotransmitters and increases plasma neurotransmitter levels (particularly dopamine and norepinephrine)^[9] but has little if any effect on mood.^[10][11][12] The effect on mood is more noticeable in humans subjected to stressful conditions (see below).

A number of studies have found tyrosine to be useful during conditions of stress, cold, fatigue,^[13] loss of a loved one such as in death or divorce, prolonged work and sleep deprivation,^[14][15] with reductions in stress hormone levels,^[16] reductions in stress-induced weight loss seen in animal trials,^[13] improvements in cognitive and physical performance^[11][17][18] seen in human trials; however, because tyrosine hydroxylase is the rate-limiting enzyme, effects are less significant than those of l-dopa.

Tyrosine does not seem to have any significant effect on mood, cognitive or physical performance in normal circumstances.^[19][20][21] A daily dosage for a clinical test supported in the literature is about 100 mg/kg for an adult, which amounts to about 6.8 grams at 150 lbs.^[22] The usual dosage amounts to 500–1500 mg per day (dose suggested by most manufacturers; usually an equivalent to 1–3 capsules of pure tyrosine). It is not recommended to exceed 12000 mg (12 g) per day. In fact, too high doses result in reduced levels of dopamine.^[19] Tyrosine may decrease the absorption of other amino acids in high or chronic doses. It decreases absorption of l-dopa.