The amino acid precursors, levodopa and L-tryptophan, were given to a group of hospitalized depressed patients in a double-blind placebo controlled study. Relatively large doses were not associated with sufficient clinical improvement to allow the patients to leave the hospital.
Previous studies using the precursor-load strategy have produced conflicting findings on the use of these compounds for depressed patients.
Mendels J, Stinnett JL, Burns D, Frazer A. Amine Precursors and Depression. Arch Gen Psychiatry. 1975;32(1):22–30. doi:10.1001/archpsyc.1975.01760190024002