L-Citrulline vs L-Arginine: Which Is Better for Blood Flow?

Short answer: In the L-citrulline vs L-arginine comparison, citrulline usually wins for blood flow. Oral arginine is heavily broken down in the gut and liver before it reaches circulation, while citrulline passes through and is converted into arginine later, so it raises blood arginine more reliably. In a controlled crossover trial, citrulline raised plasma arginine more effectively than arginine itself^[1], and in another, 7 days of citrulline (but not arginine) lowered blood pressure and improved high intensity exercise performance^[2]. Arginine still has some supportive human data, but the evidence for both is small and short term.

Key Takeaways

• Both amino acids feed the same nitric oxide (NO) pathway that helps blood vessels relax. Arginine is the direct raw material; citrulline is converted into arginine, mainly by the kidneys.
• Citrulline raised plasma arginine (AUC and peak levels) more effectively than arginine in a 20 person crossover trial^[1].
• In healthy men, 6 g/day of citrulline for 7 days lowered mean arterial pressure and improved exercise tolerance; the same dose of arginine did not^[2].
• Small sexual health studies exist for both: 1.5 g/day citrulline in a 24 man pilot^[3] and 1,500 to 5,000 mg arginine across 10 trials^[4]. Neither replaces a medical evaluation.
• Both can lower blood pressure, so anyone on blood pressure drugs, nitrates or PDE5 inhibitors should talk to a doctor first. Arginine is not recommended after a recent heart attack^[5].

What Are L-Arginine and L-Citrulline?

L-arginine is an amino acid that the body uses to build protein. Most people make enough of it on their own, and it is found in most protein rich foods^[5]. It is the substrate that nitric oxide synthase enzymes use to produce nitric oxide, the signaling molecule that tells the smooth muscle in blood vessel walls to relax. Mayo Clinic describes L-arginine as a vasodilator, meaning it helps widen blood vessels^[5].

L-citrulline is a non essential amino acid that is not used to build protein. It sits one step upstream: after absorption it is converted into arginine, which then enters the same nitric oxide pathway. Watermelon is the best known food source, but at roughly 2.3 g of citrulline per liter of juice, food alone delivers far less than the doses used in studies^[6].

Why the route matters

At first glance arginine looks like the obvious pick, since it takes the shorter path to nitric oxide. The catch is absorption. A large share of swallowed arginine is broken down in the intestine and liver (so called first pass metabolism) before it ever reaches the bloodstream^[6]. Citrulline largely escapes that breakdown, reaches the kidneys, and is turned into arginine there.

L-Citrulline vs L-Arginine: What the Human Evidence Shows

All studies below are in humans. They are small and short, so treat them as early evidence rather than proof.

Blood arginine and nitric oxide markers

The most direct comparison is a double blind, randomized, placebo controlled crossover trial in 20 healthy volunteers who took different doses of citrulline, arginine or placebo for one week each^[1]. Citrulline increased the plasma arginine AUC and peak concentration (Cmax) more effectively than arginine. At the highest citrulline dose (3 g twice daily), the arginine to ADMA ratio (a marker of how available arginine is to nitric oxide enzymes) rose from 186 to 278, urinary nitrate rose from 92 to 125 µmol/mmol creatinine, and urinary cGMP (the messenger nitric oxide uses inside vessel walls) rose from 38 to 50 nmol/mmol creatinine^[1].

One important nuance: no treatment improved flow mediated dilation (a test of artery widening) over baseline, although in the pooled data a bigger rise in the arginine/ADMA ratio tracked with better dilation^[1]. In other words, citrulline clearly improved the raw materials, but the direct effect on artery function in healthy people was modest.

Blood pressure and exercise performance

A University of Exeter crossover study gave 10 healthy adult men 6 g/day of L-arginine, 6 g/day of L-citrulline or placebo for 7 days^[2]. Both supplements raised plasma arginine by a similar amount, but only citrulline also raised plasma citrulline. Results:

• Mean arterial blood pressure was lower only after citrulline.
• During severe intensity cycling, citrulline sped up oxygen uptake kinetics (time constant 59 s on placebo vs 53 s on citrulline).
• Time to exhaustion improved from 589 s on placebo to 661 s on citrulline.
• Total work in the performance test increased slightly with citrulline (123 kJ vs 125 kJ).
• Arginine did not change oxygen kinetics, exercise tolerance or performance.

The authors concluded that short term citrulline, but not arginine, may improve blood pressure and exercise performance in healthy adults^[2]. Consumer health references are more cautious: WebMD notes that citrulline may lower blood pressure by a small amount that may not be clinically meaningful, and that its effect on performance may be small or absent in some people^[7].

Sexual function studies

Because erections depend on the same nitric oxide and cGMP signaling, both amino acids have been tested in men with mild erectile difficulties. This is where readers should be most careful, because erectile problems can be an early sign of cardiovascular disease and deserve a doctor’s evaluation.

• Citrulline: In a single blind pilot study of 24 men with mild symptoms (mean age 56.5), one month of placebo followed by one month of 1.5 g/day L-citrulline, erection hardness improved to the normal score in 12 of 24 men (50%) on citrulline vs 2 of 24 (8.3%) on placebo, and monthly intercourse frequency rose from 1.37 to 2.3. No adverse events were reported. The authors themselves described citrulline as less effective than prescription PDE5 inhibitors, at least in the short term^[3]. Examine points out that the design makes these results hard to interpret^[8].
• Arginine: A 2019 meta-analysis pooled 10 randomized controlled trials with 540 men with mild to moderate symptoms, using 1,500 to 5,000 mg of arginine. Arginine outperformed placebo or no treatment (odds ratio 3.37), with adverse effects in 8.3% of the arginine groups vs 2.3% on placebo, none severe. Doses, durations and combinations varied a lot between trials, which limits confidence^[4].

There is no large head to head trial of citrulline vs arginine for sexual function, so neither one can be called proven, and neither should be used instead of seeing a clinician.

How They Work: The Nitric Oxide Pathway in Plain Terms

The chain of events is the same for both supplements:

1. Arginine (from food, supplements or citrulline conversion) is used by nitric oxide synthase in the lining of blood vessels.
2. Nitric oxide diffuses into the smooth muscle of the vessel wall and raises cGMP.
3. cGMP relaxes the muscle, the vessel widens and blood flow increases.

Prescription PDE5 inhibitors act further down this chain by slowing the breakdown of cGMP. Examine notes that icariin from horny goat weed has also been studied for effects on PDE5^[8] (see our horny goat weed guide). Citrulline and arginine work at the start of the chain instead, by supplying more raw material.

Side by Side Comparison

How Much to Take and How to Use Them

There is no official recommended dose for either amino acid, and product labels vary. What the studies used:

• L-citrulline: 1.5 g/day split into two doses in the sexual function pilot^[3][8]; up to 3 g twice daily in the pharmacokinetic trial^[1]; 6 g/day for 6 to 7 days for exercise, with the last dose about 90 minutes before training^[6]. Oral doses as high as 15 g have been reported as well tolerated in research, but it is unclear whether more helps^[6].
• L-arginine: 1,500 to 5,000 mg per day across the sexual function trials^[4] and 6 g/day in the exercise study^[2].

Practical points:

• Check whether a product contains pure L-citrulline or citrulline malate. Citrulline malate is a blend, so the citrulline content per gram is lower; read the label for the actual citrulline amount.
• Effects in studies appeared over days to weeks, not minutes. A single dose before an event is not what most trials tested.
• Splitting larger doses through the day may reduce stomach upset.
• Choose products with independent third party testing. WebMD notes the FDA does not test these supplements to confirm they contain what the label says^[7]. Our guide to third-party tested supplements explains what to look for.

Side Effects, Safety and Who Should Talk to a Doctor

L-arginine

Mayo Clinic lists nausea, abdominal pain, diarrhea, bloating, gout, headache, allergic responses and worsened asthma as possible side effects. It is not recommended after a recent heart attack because it might increase the risk of death. It may interact with blood thinners and antiplatelet drugs (bleeding risk), blood pressure drugs, diabetes medicines, nitrates and sildenafil (blood pressure may drop too low), and potassium sparing diuretics (risk of high potassium)^[5].

L-citrulline

The most common side effect is an upset stomach, and serious allergic reactions are possible but rare. People taking blood pressure medicines, nitrates (such as nitroglycerin) or PDE5 inhibitors (such as sildenafil or tadalafil) should discuss it with a clinician first, as should anyone with heart, blood vessel, kidney or liver problems. Its effects in pregnancy are unknown^[7].

See a clinician first if

• You have heart disease, a recent heart attack, low blood pressure, or kidney or liver disease.
• You take any blood pressure medicine, nitrates, a PDE5 inhibitor, blood thinners or diabetes medicines.
• You are using either supplement because of erectile problems. These can signal circulation, blood sugar or hormone issues worth checking; see also our overview of the signs of low testosterone.

Where Ultra Bann Fits

Ultra Bann does not contain either of these amino acids. It is a natural male vitality supplement by DL Bioproducts built on seven botanicals per serving, with every amount listed on the label and no proprietary blend: Thai Butea superba 345 mg, Panax ginseng 110 mg, Cynomorium songaricum 85 mg, black ginger (Kaempferia parviflora) 85 mg, tongkat ali 85 mg, epimedium (horny goat weed) 45 mg and reishi 45 mg, for 800 mg total. The serving is one capsule every 2 days. You can read the full ingredient label or learn more about Panax ginseng for men. If you plan to combine any herbal formula with citrulline or arginine, check with your doctor, especially if you take blood pressure medicine. First orders ship free in the US and come with a 60-day money-back guarantee at the Ultra Bann shop.

Frequently Asked Questions

Is L-citrulline better than L-arginine?

For raising blood arginine, yes in the available studies. Citrulline raised plasma arginine more effectively than arginine in a controlled crossover trial^[1], and at 6 g/day it lowered blood pressure and improved exercise performance where arginine did not^[2]. Arginine has its own small body of supportive trials, so “better” depends on the goal and on the person.

Can you take L-arginine and L-citrulline together?

Many products combine them, since citrulline is converted into arginine. The trials summarized here mostly tested each one separately, so there is limited data on the combination. Because both can lower blood pressure, check with a clinician if you take any heart or blood pressure medication.

How long does L-citrulline take to work?

In the studies, effects were measured after about one week (exercise and blood markers)^[1][2] or one month (the sexual function pilot)^[3]. It is a daily supplement, not an on demand product.

Does L-arginine raise blood pressure or lower it?

It tends to lower it. Mayo Clinic notes oral L-arginine may lower blood pressure and warns it can drop too low when combined with blood pressure drugs, nitrates or sildenafil^[5].

References

1. Schwedhelm E, Maas R, Freese R, et al. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. Br J Clin Pharmacol. 2008;65(1):51-59. Abstract
2. Bailey SJ, Blackwell JR, Lord T, et al. L-citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans. Journal of Applied Physiology. 2015. University of Exeter repository
3. Cormio L, De Siati M, Lorusso F, et al. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction. Urology. 2011;77(1):119-122. doi:10.1016/j.urology.2010.08.028. Abstract
4. Rhim HC, Kim MS, Park YJ, et al. The potential role of arginine supplements on erectile dysfunction: a systemic review and meta-analysis. J Sex Med. 2019;16(2):223-234. doi:10.1016/j.jsxm.2018.12.002. Abstract
5. Mayo Clinic. L-arginine. mayoclinic.org
6. Jeukendrup A. Citrulline, arginine and performance. mySportScience. mysportscience.com
7. WebMD. L-Citrulline: uses, side effects and interactions. webmd.com
8. Examine. Can citrulline help improve symptoms of erectile dysfunction? examine.com

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease. This article is for information only and is not medical advice.