Leucine is well tolerated at the doses in most trials, and its proposed upper limit sits near 35 g a day for 70 kg. The real cautions are narrower: 1 rare disease, diabetes medicines, and very high doses.
This page sets each caution beside the evidence for it.
Quick Answer: What Are the Side Effects of Leucine?
At typical supplement doses, reported side effects are uncommon. Very high intakes, above about 500 mg per kg a day, raised blood ammonia in short studies. Taken with sugar, leucine boosts insulin release, so diabetes medicines need care. People with maple syrup urine disease must not take it at all.
Key Takeaways
- Maple syrup urine disease affects about 1 in 185,000 infants.
- People with MSUD need lifelong diets restricting all 3 branched-chain amino acids.
- Leucine taken with glucose raised the insulin response by 66%.
- Ammonia rose above normal beyond 500 mg per kg per day.
- Trials of 7.5 g daily, up to 6 months, found no glycemic harm.
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Maple Syrup Urine Disease: The Absolute No
Maple syrup urine disease, or MSUD, is an inherited disorder in which the body cannot break down leucine, isoleucine and valine. Leucine is the one that builds up most dangerously, and it can harm the brain. Anyone with MSUD must avoid leucine supplements entirely.
- Frequency: About 1 in 185,000 infants worldwide.
- Higher risk: About 1 in 380 newborns in Old Order Mennonite communities.
- Cause: A faulty enzyme complex, from changes in 3 possible genes.
- Treatment: A lifelong diet limiting branched-chain amino acids.
MedlinePlus gives the frequency figures.[1]Maple syrup urine disease — MedlinePlus Genetics, National Library of Medicine View source Leucine build-up drives the acute crises.[2]Management of acute metabolic decompensation in maple syrup urine disease: guidance based on international clinical practice — Orphanet Journal of Rare Diseases (2026) View source
The benefits side of the ledger is in the broader case for and against leucine.
Blood Sugar and Diabetes Medicines
Leucine on its own barely moves blood sugar. Taken with glucose, it raised the insulin response by 66% and halved the glucose rise in 13 healthy adults. On top of insulin or tablets that raise insulin, that effect could add up.
- Leucine alone: Little effect on glucose or insulin.
- Leucine with glucose: Insulin response up 66%, glucose rise down 50%.
- Over months: 7.5 g a day did not change HbA1c in older men with diabetes.
- Practical step: Check glucose more often when starting, as your prescriber advises.
The glucose study tested a leucine dose equal to a high-protein meal.[3]Leucine, when ingested with glucose, synergistically stimulates insulin secretion and lowers blood glucose — Metabolism (2008) View source The 6-month trial found HbA1c of 7.1% against 7.2% on placebo.[4]Prolonged leucine supplementation does not augment muscle mass or affect glycemic control in elderly type 2 diabetic men — Journal of Nutrition (2011) View source
MSKCC lists additive blood-sugar lowering with insulin and other diabetes medicines.[5]Leucine: integrative medicine monograph — Memorial Sloan Kettering Cancer Center View source Modest amounts are set out in how much leucine per day is sensible.
Very High Intakes: What Was Measured
Safety limits come from short studies that raised leucine step by step in volunteers. Above 500 mg per kg a day, blood ammonia climbed above normal and leucine spilled into urine.
- Young men: Ammonia rose beyond 500 mg per kg a day.
- Older men: Ammonia above normal beyond 550 mg per kg a day.
- Liver enzyme ALT: No change at any dose tested in young men.
- Glucose: Fell slightly but stayed in the normal range.
The young-men study ran 7 steps up to 1250 mg per kg.[6]Determination of the tolerable upper intake level of leucine in adult men — Journal of Nutrition (2012) View source The older-men study ran 8 study days in 6 men.[7]Determination of the safety of leucine supplementation in healthy elderly men — Amino Acids (2016) View source
For scale, our own leucine capsules are 1 g each, a 35th of the cautious limit for a 70 kg adult.
Kidneys, Liver and Protein Restriction
People ask about kidneys more than anything else. The trials were mostly in healthy volunteers, so there is little direct data for kidney disease, and Cleveland Clinic advises talking to a provider first.
- Healthy adults: Kidney outcomes were not the focus of most trials.
- Kidney disease: Ask first, especially on a protein-restricted diet.
- Older kidney patients: 4.5 g of leucine a day in a BCAA mix, no extra adverse events.
- Liver: ALT unchanged in the upper-limit study in young men.
The kidney trial ran 12 weeks in 55 patients over 65.[8]Anti-sarcopenic effect of leucine-enriched branched-chain amino acid supplementation among elderly chronic kidney disease patients — International Urology and Nephrology (2025) View source The provider advice is Cleveland Clinic's.[9]Getting Your Leucine: What It Does and Foods To Eat — Cleveland Clinic View source
Older Adults, Pregnancy and Children
Older adults are the group leucine is most often sold to, and the safety data there are reasonable at modest doses. Pregnancy and childhood are different: we found no trials of leucine supplements, so the default is to avoid them.
- Older men: A cautious upper limit of about 24.5 g a day at 70 kg.
- Frail older women: 7.5 g a day for 12 weeks, insulin sensitivity unchanged.
- Pregnancy and breastfeeding: No supplement trials found.
- Children: Not a supplement population; ask a paediatrician.
The 24.5 g figure is the lower bound of the older-men estimate.[10]Safety and Tolerability of Leucine Supplementation in Elderly Men — Journal of Nutrition (2016) View source The frail women trained 3 times a week.[11]Leucine Supplementation Does Not Alter Insulin Sensitivity in Prefrail and Frail Older Women following a Resistance Training Protocol — Journal of Nutrition (2019) View source
The benefit evidence for this age group is in leucine for adults over 65.
Other Reported Effects
Several claims circulate with little behind them. MSKCC notes that excessive leucine may interfere with vitamins B3 and B6, a concern from high-leucine diets rather than capsule doses.
- Vitamins B3 and B6: Possible interference at very high intakes, per MSKCC.
- Digestive upset: Often listed online, rarely measured in trials.
- Cramps or headaches: No trial evidence that we could find.
- Allergy: Not reported in the trials we read.
If a reaction appears after starting any supplement, stop it and note the dose and timing for your clinician.
Stop and Speak to a Clinician If
- You have maple syrup urine disease or any other disorder of leucine metabolism
- You take insulin or another medicine that lowers blood sugar
- You have kidney or liver disease, or follow a protein-restricted diet
- You are pregnant or breastfeeding, where no leucine supplement trials exist
- You are buying it for a child or a teenager
Frequently Asked Questions
Is L-leucine hard on the kidneys? +
There is little evidence of harm in healthy people, but kidney disease was rarely studied. In 1 trial, older kidney patients took 4.5 g of leucine a day for 12 weeks without extra adverse events.
Who shouldn't take L-leucine? +
Anyone with maple syrup urine disease, about 1 in 185,000 infants. Also check first if you take diabetes medicine, have kidney disease, or are pregnant.
What are the side effects of taking leucine? +
Few at supplement doses. Above about 500 mg per kg a day, blood ammonia rose in short studies, and leucine with sugar raises insulin release.
Does leucine interact with any medications? +
The main one is diabetes medicine: leucine with glucose raised insulin release by 66%. MSKCC also lists a possible sildenafil interaction of unknown relevance, plus 2 B vitamins.
Is leucine safe for long-term use? +
The longest trial we found ran 6 months at 7.5 g a day without changes in blood sugar control or lipids. Multi-year data do not exist.
Can leucine cause low blood sugar? +
On its own it barely moved glucose in 13 adults. With glucose it cut the glucose rise by 50%, so watch for lows if you use insulin.
What does MSUD smell like? +
It is named after the sweet, maple-syrup smell of affected babies' urine. The classic form shows soon after birth and affects about 1 in 185,000 infants.
Which supplement would someone with maple syrup urine disease take? +
Specialist medical formulas free of all 3 branched-chain amino acids, under a metabolic team, never leucine. Valine and isoleucine are sometimes given in measured amounts during crises.
Can you take leucine while pregnant? +
We found 0 trials of leucine supplements in pregnancy. Food protein is the safe source; ask your obstetrician before any amino acid supplement.
Related Reading
- Muscle Protein Synthesis Versus Muscle Gained
- Getting Leucine From Food First
- How Leucine Compares With HMB and Whey
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