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Serotonin Precursor Dietary Supplement Plant Based L-Tryptophan The stress of living in a fast-paced world with demands of work and family may sometimes leave you feeling tense, anxious and unable to be your best. Perhaps you've got too much on your mind, affecting your sleep, confidence and concentration. L-Tryptophan is an essential amino acid that serves as a precursor to the important neurotransmitter serotonin.