L-citrulline is an amino acid that raises blood levels of arginine more effectively than arginine supplements themselves, and arginine is the raw material your body uses to make nitric oxide, a molecule that relaxes blood vessels. That single mechanism feeds into a surprisingly wide list of studied benefits, from lower blood pressure and reduced arterial stiffness to better exercise tolerance, less post-workout soreness, and improved erectile function. The evidence is strongest in some of these areas and still developing in others, and the form you take matters more than you might expect.
Why Citrulline Works Better Than Taking Arginine Directly
When you swallow an arginine supplement, much of it gets broken down by enzymes in the gut and liver before it ever reaches your bloodstream. Citrulline sidesteps that problem. It passes through the digestive tract mostly intact, enters the kidneys, and gets converted into arginine there, which then circulates freely. A pharmacokinetic study in healthy volunteers found that oral citrulline raised plasma arginine levels more effectively than equivalent or even double doses of oral arginine.1PubMed Central. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism A clinical pharmacology review confirmed that citrulline’s advantage comes from avoiding first-pass metabolism in the liver and having a longer circulation time.2PubMed Central. Therapeutic Potential of Citrulline as an Arginine Supplement: A Clinical Pharmacology Review In animal modeling, citrulline’s oral bioavailability was estimated at about 82% compared to roughly 53% for arginine, and nearly all of the citrulline that entered the bloodstream was converted to arginine.3PubMed Central. Quantitative characterization of L-citrulline as a precursor of L-arginine in rats: pharmacokinetic basis for nitric oxide-mediated effects
Once that arginine reaches endothelial cells lining your blood vessels, an enzyme called endothelial nitric oxide synthase uses it to produce nitric oxide. Nitric oxide signals the smooth muscle in vessel walls to relax, which widens blood vessels and lowers resistance to blood flow. In lab studies on porcine coronary arteries, citrulline restored nitric oxide production and vessel relaxation that had been impaired by a naturally occurring inhibitor, while also increasing the expression of the enzymes involved in that process.4PubMed Central. L-citrulline for protection of endothelial function from ADMA–induced injury in porcine coronary artery This is the core mechanism behind most of citrulline’s studied benefits. More arginine availability means more nitric oxide, and more nitric oxide means wider, more flexible blood vessels.
Blood Pressure and Arterial Stiffness
Stiff arteries are one of the early markers of cardiovascular risk, and they tend to get worse with age. A placebo-controlled trial in middle-aged adults found that citrulline supplementation significantly reduced arterial stiffness, as measured by pulse wave velocity, compared to placebo.5International Journal of Cardiology. Short-term effects of l-citrulline supplementation on arterial stiffness in humans Interestingly, that particular study did not find significant blood pressure changes despite the improvement in arterial flexibility, which suggests the two measures do not always move in lockstep.
More recent work in people with type 2 diabetes paints a broader picture. Four weeks of citrulline supplementation improved flow-mediated dilation (a measure of how well arteries expand in response to increased blood flow), reduced arterial stiffness in the legs, and lowered aortic systolic blood pressure both in a fasted state and during acute blood sugar spikes.6PubMed Central. Effects of L-Citrulline Supplementation on Endothelial Function, Arterial Stiffness, and Blood Glucose Level in the Fasted and Acute Hyperglycemic States in Middle-Aged and Older Adults with Type 2 Diabetes The blood pressure benefits seem more consistent in populations that already have vascular impairment, such as people with diabetes or older adults, than in young healthy people with normal vessel function. If your arteries are already in good shape, there may be less room for improvement.
Exercise Performance
This is where the evidence gets complicated, and where the supplement industry’s claims run ahead of the science. Citrulline is one of the most popular pre-workout ingredients, typically marketed as a way to increase endurance, delay fatigue, and improve “the pump.” The reality is more mixed than that.
For aerobic exercise, a 2022 systematic review and meta-analysis pooling data from multiple trials found no statistically significant benefit of citrulline supplementation on aerobic performance, perceived exertion, oxygen uptake kinetics, or blood lactate levels.7PubMed Central. Effects of Citrulline Supplementation on Different Aerobic Exercise Performance Outcomes: A Systematic Review and Meta-Analysis That is a fairly definitive verdict from pooled data. An individual trial did report improved time to exhaustion and faster oxygen uptake during severe-intensity cycling after a week of citrulline supplementation.8PubMed. l-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans But when multiple studies are combined and the results averaged out, the overall effect on aerobic performance is essentially zero. If you are a runner or cyclist hoping citrulline will shave time off your performance, the best current evidence says it probably will not.
For resistance training, the picture looks somewhat more favorable. A six-week double-blind trial in resistance-trained men compared pure L-citrulline, citrulline malate, and placebo. Both citrulline forms led to significantly more total upper-body repetitions to failure compared to placebo, with the L-citrulline group gaining about 5.6 more total reps and the citrulline malate group gaining about 4.5 more reps over the study period.9PubMed Central. Changes in resistance training performance, rating of perceived exertion, and blood biomarkers after six weeks of supplementation with L-citrulline vs. L-citrulline DL-malate in resistance-trained men: a double-blind placebo-controlled trial A few extra reps per session may sound modest, but over weeks and months of training, that accumulated extra volume can contribute to measurably greater strength and muscle gains. The effect seems more reliable for high-rep, fatigue-dependent work than for maximal-strength efforts like a one-rep max.
Muscle Soreness After Training
One of citrulline’s more consistent findings is a reduction in delayed-onset muscle soreness, the aching you feel a day or two after a hard workout. A systematic review and meta-analysis of seven studies found that citrulline supplementation significantly reduced muscle soreness ratings at 24 hours post-exercise, with a large effect size.10PubMed Central. Effect of citrulline on post-exercise rating of perceived exertion, muscle soreness, and blood lactate levels: A systematic review and meta-analysis The heterogeneity across studies was high, meaning individual results varied a lot, but the overall direction was clear.
A trial in sub-elite athletes tracked soreness at multiple time points after intense exercise and found the citrulline group had meaningfully lower pain scores at 72 hours post-exercise compared to placebo.11Retos. The effect of citrulline on delayed onset muscle soreness (DOMS) and aerobic and anaerobic endurance in sub elite athletes The mechanism likely ties back to improved blood flow. Better circulation means faster clearance of metabolic byproducts from working muscles and more efficient delivery of nutrients needed for repair. For anyone who trains hard and dreads being hobbled the next day, this is arguably citrulline’s most practically relevant exercise benefit.
Erectile Function
The connection between citrulline and erections follows logically from the nitric oxide pathway. Erections depend heavily on nitric oxide signaling to relax smooth muscle in penile blood vessels, which is why drugs that boost nitric oxide signaling (like PDE-5 inhibitors) are the standard treatment for erectile dysfunction. Citrulline works upstream of that same pathway by increasing the supply of arginine available for nitric oxide production.
A small but well-designed crossover trial gave 24 men with mild erectile dysfunction either 1.5 grams of L-citrulline per day or placebo for one month each. Half the men on citrulline improved from a mild-ED erection hardness score to a normal score, compared to only about 8% on placebo. The average number of intercourses per month nearly doubled during the citrulline phase.12PubMed. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction All the men who improved reported being very satisfied. The caveat is that this was a small study and the dose was relatively low. It is not a replacement for prescription medications in moderate-to-severe ED, but for mild cases, citrulline is a reasonable option with a plausible mechanism and almost no side effects.
Blood Sugar and Metabolic Health
Citrulline’s vascular benefits overlap with metabolic health in people with type 2 diabetes. In a study of 25 patients with type 2 diabetes who took 2,000 mg of citrulline daily for one month, the enzyme arginase (which competes with nitric oxide synthase for arginine) dropped by about 21%, while plasma nitric oxide levels increased by about 38%.13PubMed Central. L-Citrulline Supplementation Increases Plasma Nitric Oxide Levels and Reduces Arginase Activity in Patients With Type 2 Diabetes In plain terms, citrulline shifted the competition for arginine away from waste and toward blood vessel protection.
Separate work in middle-aged and older adults with type 2 diabetes found that four weeks of citrulline supplementation improved endothelial function, reduced arterial stiffness, lowered aortic systolic blood pressure, and brought down blood glucose levels in both fasted and sugar-challenged states.14PubMed Central. Effects of L-Citrulline Supplementation on Endothelial Function, Arterial Stiffness, and Blood Glucose Level in the Fasted and Acute Hyperglycemic States in Middle-Aged and Older Adults with Type 2 Diabetes The glucose-lowering effect is still being explored, but may relate to improved blood flow to skeletal muscle, which is the primary site of glucose disposal after meals. Better perfusion of working muscle could mean better glucose uptake. This is a developing area of research and not yet a basis for clinical recommendations, but it is worth watching.
Aging, Muscle Loss, and Sarcopenia
Sarcopenia, the age-related loss of muscle mass and strength, is one of the biggest threats to independence in older adults. Citrulline has attracted interest here for two reasons. First, it may directly stimulate muscle protein synthesis under conditions of nutritional stress. Cell culture studies found that citrulline boosted protein synthesis by roughly 22% under amino acid deficiency and about 11% under energy stress, outperforming both leucine and arginine under the energy-stressed condition.15PubMed Central. Citrulline stimulates muscle protein synthesis, by reallocating ATP consumption to muscle protein synthesis The proposed mechanism is that citrulline redirects a greater share of the cell’s energy budget toward building proteins.
In human trials, supplementation with 10 grams of citrulline in malnourished elderly patients improved amino acid availability and increased lean mass and appendicular skeletal muscle mass, though the lean-mass effect was observed only in women. In obese elderly patients, combining citrulline with high-intensity exercise decreased fat mass and increased muscle strength compared to exercise alone. Even lower doses of citrulline malate (around 3 grams) improved walking speed in older adults.16Journal of Microbiology and Biotechnology. The Effects of Protein and Supplements on Sarcopenia in Human Clinical Studies: How Older Adults Should Consume Protein and Supplements A separate study in sarcopenic elderly men found that high-intensity interval training combined with citrulline supplementation improved muscle quality, walking speed, and lean mass index.17Journal of Sports and Biomotor Sciences. Neuromuscular indices adaptation of sarcopenic elderly men to high-intensity interval training and L-citrulline supplementation The common thread across these trials is that citrulline paired with exercise appears more effective than either alone, which makes sense given that exercise creates the demand for new muscle protein while citrulline may improve the conditions for building it.
Immune Function and Inflammation With Aging
A 2025 study published in Science Advances explored citrulline’s effects on aging at a more fundamental level. Researchers found that citrulline reduced markers of cellular senescence, protected against DNA damage, prevented cell cycle arrest, and shifted macrophage metabolism away from inflammatory states in aging mice. The authors characterized citrulline as an “endogenous metabolite antagonist to inflammation,” meaning the body may naturally use it to counterbalance pro-inflammatory signaling, and that supplementing it could reinforce that protective role.18PubMed Central. Citrulline regulates macrophage metabolism and inflammation to counter aging in mice This is animal research and cannot be directly translated to human health recommendations, but it adds a mechanistic layer to the clinical observations of citrulline helping older adults. The chronic, low-grade inflammation that accumulates with age (sometimes called “inflammaging”) is increasingly recognized as a driver of muscle loss, cardiovascular disease, and cognitive decline. If citrulline genuinely dampens that process, it could explain some of the broad benefits seen in older populations.
Brain Blood Flow
Because nitric oxide dilates blood vessels throughout the body, not just in the periphery, researchers have begun looking at citrulline’s effects on the brain. A study in taekwondo athletes found that citrulline supplementation during sprint interval training significantly improved cerebrovascular reactivity, a measure of how well the brain’s blood vessels respond to changing demands. The citrulline group maintained their baseline brain blood-flow responsiveness, while the placebo group showed a decline.19PubMed Central. The effects of L-citrulline supplementation on cerebrovascular function during sprint interval training in taekwondo athletes This is a single study in young athletes, so it is far too early to make claims about cognitive enhancement. But the finding is consistent with the broader vascular mechanism and raises questions about whether citrulline could help preserve brain blood flow in aging or in conditions where cerebrovascular function is compromised.
Citrulline Malate vs. Pure L-Citrulline
Many pre-workout supplements use citrulline malate rather than pure L-citrulline. The malate component is a salt of malic acid, which plays a role in the energy-production cycle inside mitochondria. The idea is that malate could independently boost ATP production by keeping the energy cycle running smoothly during high-demand exercise, thereby offering a dual benefit alongside citrulline’s nitric oxide effects.20PubMed Central. A critical review of citrulline malate supplementation and exercise performance
In practice, the difference between the two forms is smaller than the marketing suggests. The six-week resistance training trial mentioned earlier found that both pure L-citrulline and citrulline malate significantly outperformed placebo for upper-body repetition performance, with no meaningful difference between the two supplement groups.21PubMed Central. Changes in resistance training performance, rating of perceived exertion, and blood biomarkers after six weeks of supplementation with L-citrulline vs. L-citrulline DL-malate in resistance-trained men: a double-blind placebo-controlled trial One practical issue with citrulline malate is that the ratio of citrulline to malate varies between products, and labels often do not make this clear. A product listing “6 grams of citrulline malate” in a 2:1 ratio actually contains only 4 grams of citrulline. If you are comparing the two forms on a label, check whether the citrulline content is equivalent. For most purposes, pure L-citrulline at the right dose gives you the active ingredient without any ambiguity about ratios.
Where Citrulline Comes From in Food
Citrulline takes its name from Citrullus lanatus, the Latin name for watermelon, which remains its richest natural food source. Eating watermelon increases plasma levels of both citrulline and arginine.22PubMed Central. Current Evidence of Watermelon (Citrullus lanatus) Ingestion on Vascular Health: A Food Science and Technology Perspective However, the concentrations in whole fruit are relatively modest. You would need to eat a substantial amount of watermelon daily to match even a low supplement dose. The rind contains more citrulline than the flesh, but few people eat watermelon rind by choice. Other cucurbits like cucumber and other melons contain smaller amounts. For anyone seeking the doses used in clinical research, typically 3 to 8 grams per day for exercise and vascular benefits, supplementation is the realistic route.
Clinical Uses in Specific Medical Conditions
Beyond the general-population benefits, citrulline has a long-standing role in managing urea cycle disorders, rare genetic conditions where the body cannot properly convert ammonia to urea for excretion. Citrulline supplementation supports ureagenesis in these patients and is routinely used before and after liver transplantation.23PubMed Central. Impact of supplementation with L-citrulline/arginine after liver transplantation in individuals with Urea Cycle Disorders A review of citrulline’s therapeutic potential noted its established role in genetic urea cycle disorders while also highlighting growing interest in applying it to other conditions where arginine depletion and nitric oxide deficiency are problems.24PubMed. Potential therapeutic uses of L-citrulline beyond genetic urea cycle disorders
Sickle cell disease is one of those conditions. Patients with sickle cell disease tend to have low arginine levels and impaired nitric oxide production, which contributes to the vascular complications of the disease. A pilot study in young sickle cell patients found that oral citrulline at about 0.1 g per kilogram of body weight per day raised plasma arginine, reduced elevated white blood cell counts toward normal, and improved subjective well-being, with no reported side effects.25PubMed Central. Oral citrulline as arginine precursor may be beneficial in sickle cell disease: early phase two results This is early-phase data, but the rationale is strong enough that larger trials are warranted.
High Altitude and Low-Oxygen Environments
At high altitudes, reduced oxygen availability causes pulmonary arteries to constrict, which can raise pulmonary arterial pressure and contribute to altitude sickness. Because nitric oxide is a potent vasodilator in the lungs, researchers have tested whether boosting the arginine-nitric oxide pathway with citrulline might help. A pilot study of arginine-citrulline supplementation during chronic intermittent hypoxia exposure found a trend toward a smaller increase in pulmonary arterial pressure compared to placebo, though the results did not reach conventional statistical significance. The authors noted that long-term compliance with the supplement regimen and individual genetic variation in nitric oxide-related enzymes may influence whether this approach works for a given person.26High altitude medicine & biology. Compliance and Genetic Variability Are Determinants of the Success of l-Arginine/l-Citrulline Supplementation in High Altitude-A Pilot Study This is an area where the biological rationale is compelling but the clinical evidence remains preliminary. If you are heading to high altitude and considering citrulline, it is unlikely to cause harm, but do not count on it as your primary altitude strategy.
Dosing Patterns and What to Expect
Doses in the research range from 1.5 grams per day at the low end (the erectile function trial) to 10 grams per day in the sarcopenia studies. For general vascular and exercise purposes, most trials use between 3 and 8 grams of L-citrulline per day, often split into two doses. Both citrulline and citrulline malate have been shown to increase plasma citrulline and arginine concentrations, along with total nitrate and nitrite levels, which are markers of nitric oxide production.27The Journal of Strength & Conditioning Research. Effects of Citrulline Supplementation on Exercise Performance in Humans: A Review of the Current Literature
Citrulline has a strong safety profile across the studies reviewed here. No trial reported significant adverse effects, even at doses up to 10 grams daily. Gastrointestinal discomfort at very high single doses is the most commonly noted concern, which is easily managed by splitting the dose. Because citrulline lowers blood pressure through vasodilation, people already taking antihypertensive medications should be aware of the possibility of additive effects, though this has not been a significant clinical problem in trials to date. People with urea cycle disorders should only use citrulline under medical supervision, since their metabolic handling of the amino acid is fundamentally different from the general population.