An inexpensive substance one step before serotonin, sold without prescription. Best supported for low mood and for appetite and body weight; weaker signals for poor sleep, thinking in older adults and widespread muscle pain. Many trials are small, old and never independently repeated. Nausea and diarrhoea are common; the main hazard is combining it with anything that raises serotonin. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Absolute eosinophil count | Under 300 cells/µL | Detects the eosinophilia that precedes eosinophilia-myalgia syndrome |
| Serum tryptophan | 45-80 µmol/L | Indicates whether precursor supply is genuinely low before adding a downstream precursor |
| High-sensitivity C-reactive protein | Under 1.0 mg/L | Inflammation diverts tryptophan away from serotonin, predicting who has headroom to gain |
| Serum vitamin B6 (pyridoxal-5'-phosphate) | 30-80 nmol/L | The cofactor required to convert 5-HTP into serotonin |
| Alanine aminotransferase (ALT) | 10-26 U/L (women), 10-33 U/L (men) | Confirms intact clearance capacity before and during use |
| 24-hour urinary 5-hydroxyindoleacetic acid (5-HIAA) | No established target under supplementation; track against the individual's own pre-supplement value | Confirms systemic serotonin turnover has risen, and flags the false-positive carcinoid result |
| Resting heart rate and blood pressure | Under 70 bpm resting; under 120/80 mmHg | Rising values with tremor or agitation are the earliest bedside sign of serotonin excess |
Cadence: Baseline before starting; blood count repeated at 8-12 weeks, then every 6-12 months where use continues; chosen outcome score at 4 weeks and 12 weeks