A protein building block the body cannot make for itself, used to produce the messengers most tied to sleep and mood. Its clearest measured effect is staying asleep rather than falling asleep, at a gram or more near bedtime, away from food. Next-day sedation is common; the 1989 outbreak of a painful muscle and immune illness keeps safety open. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Absolute eosinophil count | Below 350 cells/µL | Earliest signal of the immune reaction behind eosinophilia-myalgia syndrome |
| Plasma tryptophan | 45–80 µmol/L | Shows whether a low starting level leaves room for supplementation to help |
| Kynurenine-to-tryptophan ratio | Below roughly 30 µmol/mmol | Shows how much of a dose is diverted down the inflammatory branch instead of the serotonin branch |
| High-sensitivity C-reactive protein | Below 1.0 mg/L | Inflammation drives tryptophan away from serotonin and toward kynurenine |
| Serum pyridoxal 5'-phosphate | 50–100 nmol/L | The active vitamin B6 form is required for the steps that make serotonin and niacin |
| Serum 25-hydroxyvitamin D | 40–60 ng/mL | Vitamin D regulates the gene for the brain enzyme that makes serotonin |
| Alanine aminotransferase | 10–26 U/L in men, 7–22 U/L in women | Liver health governs the enzyme that clears most of an oral dose |
Cadence: Baseline blood draw before starting; complete blood count with differential repeated at 4 weeks and 3 months, then every 6–12 months while use continues; tryptophan and kynurenine measures repeated only if the response is disappointing.