L-Ornithine

Supplements · Amino Acids · Supplement, Typed Supps

A non-essential amino acid your body makes itself and uses to help clear ammonia — studied in small trials for exercise fatigue and, separately, for stress and sleep, with genuinely interesting but limited results so far.

Stage by stage

Intake (Oral capsule): Timing varies by intended use.
Small intestine (Absorbed): Absorbed and distributed via the blood.
Bloodstream (Circulates): Travels to the liver and other tissues.
Liver (Urea cycle): Used to help convert ammonia into urea for safe excretion.

What the evidence supports

Reducing ammonia buildup or perceived fatigue during intense exercise — limited evidence. Small studies in athletes report reduced ammonia buildup or modestly better repeated-sprint performance with 2–6 g doses, but other studies with different protocols find no benefit — results depend heavily on dose, timing, and training status, and independent replication is limited.
Improving subjective sleep quality and lowering the cortisol/DHEA-S stress ratio at low daily doses — limited evidence. A randomized, placebo-controlled trial in 52 stressed Japanese workers found 400 mg/day for 8 weeks reduced serum cortisol and the cortisol/DHEA-S ratio and improved perceived sleep quality — a genuinely interesting result, but from essentially one trial in one population.
Evidence on General short-term oral tolerability — moderate evidence. Mild GI upset (nausea, cramping) has been reported at higher doses in the small trials available; no long-term safety data exists at either the low sleep-focused doses or the higher exercise-focused doses.

Typical dose

Sleep/stress trials have used around 400 mg/day; exercise-focused trials have used a much wider 2–6 g/day range

Upper limit: No established UL

Timing

For sleep/stress use, typically a low dose in the evening; for exercise use, timing varies by study.

Time to effect

Weeks: The stress/sleep trial measured benefit after 8 weeks of consistent daily use.

Common myth

“Ornithine works like arginine or citrulline to boost nitric oxide and blood flow.” In reality: Ornithine’s well-studied role is in ammonia clearance via the urea cycle — a different mechanism from arginine and citrulline’s nitric-oxide pathway, so “pump” claims for ornithine specifically aren’t well supported.