What happens when you take l-arginine with l-citrulline?
Both of these amino acids feed the same pathway: the body uses L-arginine to make nitric oxide, the signaling molecule that relaxes blood vessels. They simply arrive at that pathway by different routes, which is why combining them behaves differently from taking either one alone.
- L-arginine gives a fast but short peak. Oral L-arginine is the direct substrate for nitric oxide synthase, so plasma arginine rises quickly. But a substantial portion of each dose is broken down by arginase in the gut wall and by first-pass metabolism in the liver, so the peak fades within an hour or two.
- L-citrulline takes the back door. L-citrulline is poorly metabolized in the gut and liver, absorbs efficiently, and is then converted into L-arginine in the kidneys via the urea cycle. This sidesteps the first-pass loss, so it raises plasma arginine higher and for longer than the same amount of arginine taken directly.
- Together you get a peak plus a plateau. In a human crossover study, the combination raised plasma arginine more than either amino acid given alone, producing a quick rise from the arginine and a more sustained level from the kidney-derived arginine that citrulline supplies.
Why is this important?
Nitric oxide availability tends to decline with age and with conditions such as high blood pressure, type 2 diabetes, and cardiovascular disease. Supporting nitric oxide production has been linked with better endothelial (blood-vessel lining) function, blood-pressure support, and improved exercise capacity, so the way these two amino acids reach the pathway matters for how reliably they work.
Practically, the combination is appealing for two situations. For everyday cardiovascular support, the sustained arginine level from citrulline keeps the pathway supplied beyond the short arginine peak. For exercise, a small randomized crossover trial in athletes found the combination raised arginine and a marker of nitric oxide and improved high-intensity cycling performance, which is why pre-workout formulas often pair them.
It also matters for safety. The same vasodilating effect that makes these useful is exactly what can become a problem when stacked with medicines that already lower blood pressure or dilate vessels. That is the main reason to be deliberate about combining them rather than assuming "more is better."
What should you do?
This is a low-risk, generally well-tolerated combination for most healthy adults. Use a simple schedule rather than chasing precise gram targets, and confirm the right amounts for you with a clinician.
- Before you start: If you take nitrates (such as nitroglycerin or isosorbide), PDE5 inhibitors (such as sildenafil or tadalafil), or blood-pressure medication, check with your doctor or pharmacist first. Additive vasodilation can drop blood pressure too far. If you are prone to herpes simplex flares, be aware arginine may trigger them in susceptible people.
- Every day: Take the pair on an empty stomach for best absorption. Splitting the daily intake into a morning and an evening portion helps keep plasma levels steadier. For pre-workout use, take it roughly 30 to 60 minutes before training so the peak lands during your session.
- After any change: If you start a new medication, especially one for blood pressure or erectile function, revisit whether to keep taking the combination. Stop and seek advice if you notice lightheadedness, dizziness on standing, or unusual flushing, which can signal blood pressure dropping too low.
Which specific products are affected?
You will most often see these two together in pre-workout and "nitric oxide booster" supplements. Common forms include L-arginine HCl, AAKG (arginine alpha-ketoglutarate), citrulline malate (citrulline bound to malic acid), and plain L-citrulline, sometimes alongside beetroot powder, which provides nitrates through a separate route.
Favor products that disclose the amount of each amino acid rather than burying them in a proprietary blend, because an underdosed citrulline component is unlikely to do much. Be cautious about stacking these with separate nitric-oxide boosters such as agmatine or with beetroot/nitrate products at the same time, since the combined effect on blood pressure is harder to predict.
The science behind it
The combination has direct human evidence rather than just theory. In a crossover trial in healthy men, combined oral L-citrulline and L-arginine raised plasma L-arginine more than either supplement taken alone (Suzuki T, et al. Biosci Biotechnol Biochem. 2017;81(2):372-375).
A randomized, double-blind, placebo-controlled crossover trial in male collegiate soccer players found that the same combination improved performance on a 10-minute full-power cycling test, alongside increases in plasma arginine and nitric oxide markers (Suzuki I, et al. PMC6469824).
The underlying pharmacology was characterized earlier: an oral crossover pharmacokinetic study showed that citrulline raises plasma arginine and supports nitric oxide metabolism, while oral arginine is subject to heavy first-pass loss (Schwedhelm E, et al. Br J Clin Pharmacol. 2008;65(1):51-59; PMC2291275). That study tested each amino acid alone, which is why the two combination trials are the better basis for the synergy claim.
Frequently Asked Questions
Is taking L-arginine and L-citrulline together dangerous?
For most healthy adults, no. It is a low-risk, well-tolerated combination. The main caution is not the pair itself but combining it with medicines that already lower blood pressure or dilate blood vessels.
Why combine them instead of just taking more L-arginine?
Because a large share of oral L-arginine is destroyed before it reaches your bloodstream. Citrulline bypasses that loss and tops up arginine over a longer window, so the pair sustains higher plasma arginine than arginine alone.
When should I take the combination?
On an empty stomach for best absorption. For exercise, take it roughly 30 to 60 minutes before training; for general support, splitting it across morning and evening keeps levels steadier.
Can I take this with my blood pressure or erectile dysfunction medication?
Not without checking first. Nitrates, PDE5 inhibitors (such as sildenafil or tadalafil), and blood-pressure drugs all lower blood pressure or dilate vessels, and adding these amino acids can push that effect too far. Ask your doctor or pharmacist.
Does citrulline malate count as L-citrulline?
Citrulline malate is L-citrulline bound to malic acid, so only part of its weight is citrulline. It is the most-studied pre-workout form, but read the label to see how much actual citrulline you are getting.
Could arginine cause a herpes flare?
In people prone to herpes simplex, arginine may trigger outbreaks. If that applies to you, raise it with your clinician before using arginine-containing products.
Key takeaways
- L-arginine gives a fast, short-lived rise in plasma arginine; L-citrulline gives a slower, more sustained one by bypassing first-pass loss. Together they raise arginine higher and for longer than either alone.
- Human trials support both the plasma-arginine effect and an exercise-performance benefit, so the synergy is real but modest, not dramatic.
- Take on an empty stomach; confirm the right amounts with a clinician rather than chasing precise gram targets.
- The main safety point is drug interactions: avoid combining with nitrates, PDE5 inhibitors, or aggressive blood-pressure medicines without medical advice.