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Limited evidenceStress & mood

L-Ornithine

L-ornithine has a coherent biochemical story around ammonia clearance and a couple of small positive Japanese trials for fatigue and sleep, but the total human evidence amounts to a few dozen participants.

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What the science says

  • Reduced subjective fatigue in a small crossover trial Limited evidence

    A randomized crossover study in 17 healthy volunteers found 2 g/day of L-ornithine for one week reduced subjective fatigue after a cycling test and shifted markers of ammonia detoxification and lipid metabolism. With fewer than 20 participants and no independent replication, this is preliminary.

  • Modest improvements in stress markers and sleep in office workers Limited evidence

    A randomized controlled trial in 52 healthy workers found 400 mg/day of L-ornithine for 8 weeks lowered the serum cortisol to DHEA-S ratio and improved several sleep quality subscales versus placebo. It was a single-site trial with self-reported sleep outcomes and no objective sleep measurement.

  • Do not confuse it with L-ornithine L-aspartate for liver disease Moderate evidence

    The strong ammonia-lowering evidence in hepatic encephalopathy relates to L-ornithine L-aspartate, usually given intravenously in hospital, not to over-the-counter L-ornithine capsules. That clinical literature does not transfer to healthy people taking a gram or two orally.

  • No convincing performance or growth hormone effect Limited evidence

    Ornithine's popularity in bodybuilding rests on studies where very large oral doses transiently raised growth hormone, an effect that has never been shown to translate into strength or muscle gains. Doses high enough to move GH also reliably cause diarrhea.

Dosage & safety

Studied doseFatigue trials used about 2 g/day for 7 days; the stress and sleep trial used 400 mg/day for 8 weeks. Ammonia-focused exercise protocols have used 6-12 g/day, with poor GI tolerance.
SafetyDoses above roughly 10 g/day commonly cause osmotic diarrhea, cramping and nausea; below 3 g/day it is generally well tolerated. Because ornithine feeds the urea cycle, people with urea cycle disorders, cirrhosis or advanced kidney disease should not supplement without medical advice. No significant drug interactions are established, but data are sparse. There is no meaningful safety information for pregnancy, breastfeeding or use beyond a few months.

How to take it

TimingBefore bed in the stress and sleep protocol; otherwise timing does not matter.
With food?With or without food; a meal helps at multi-gram doses.
Worth knowingAbove roughly 10 g/day it acts as an osmotic laxative, so there is no benefit to pushing the dose upward.

Frequently asked questions

Does L-ornithine reduce fatigue?

The evidence is limited: a crossover study in 17 volunteers found 2 g/day of L-ornithine for a week reduced fatigue after cycling, but it has not been independently replicated.

Does L-ornithine help sleep or stress?

The evidence is limited: one trial in 52 office workers found 400 mg/day of L-ornithine for 8 weeks lowered a stress hormone ratio and improved self-reported sleep, without objective sleep measurement.

Does ornithine boost growth hormone for muscle?

No convincing evidence — it is graded limited. Very large ornithine doses transiently raised growth hormone, but never translated into strength or muscle gains, and those doses reliably cause diarrhea.

Is L-ornithine the same as L-ornithine L-aspartate for the liver?

No. The ammonia-lowering evidence in liver disease is for L-ornithine L-aspartate, usually given intravenously in hospital, and it does not transfer to over-the-counter L-ornithine capsules.

Scientific references

Where to buy

Nutricost, L-Ornithine, 500 mg, 180 Capsules

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