L-tyrosine is an amino acid that serves as a precursor for the synthesis of catecholamine neurotransmitters, including dopamine, norepinephrine, and epinephrine. These neurotransmitters are involved in mood regulation and cognitive function, and their dysregulation is implicated in depressive disorders. The rationale for using L-tyrosine to support or treat depression is based on its role in neurotransmitter biosynthesis, particularly in situations where increased demand or stress may deplete catecholamine stores.

However, the scientific evidence supporting L-tyrosine supplementation for depression is limited and largely indirect. Most clinical studies have focused on its effects in mitigating cognitive decline during acute stress, sleep deprivation, or extreme environmental conditions, rather than directly treating major depressive disorder. A few small studies and case reports suggest possible benefit in individuals with dopamine-related depression or in those with low phenylalanine/tyrosine levels, but large, well-controlled trials in depressed populations are lacking. Systematic reviews and meta-analyses generally conclude that there is insufficient evidence to recommend L-tyrosine as a treatment for depression.

In summary, while the biochemical basis for L-tyrosine's use in depression is plausible, and there is some scientific exploration, robust clinical validation is lacking. It is not a standard or first-line treatment for depression, and its efficacy in this context remains unproven.