Watermelon contains an amino acid called L-citrulline that your body converts into nitric oxide, the same molecule that prescription erectile dysfunction drugs work through. A small but real clinical trial found that daily L-citrulline supplements improved erection hardness in half of men with mild ED, compared to about 8 percent on placebo. The catch is that the evidence base remains thin, and the amount of citrulline in a typical serving of watermelon falls well short of what those studies used. The story is more nuanced than either “nature’s Viagra” or “just a fruit.”
How Watermelon Connects to Erections
Erections depend heavily on blood flow. When nitric oxide is released inside the blood vessels of the penis, smooth muscle in those vessel walls relaxes, allowing blood to rush in and fill the erectile tissue. This is the same basic mechanism that sildenafil and similar drugs exploit: they prevent the breakdown of a chemical messenger that nitric oxide triggers, keeping those blood vessels dilated longer.
Watermelon is one of the richest food sources of L-citrulline, an amino acid that feeds into the production of nitric oxide. Once absorbed, L-citrulline travels to the kidneys and gets converted into L-arginine, which is the direct raw material that cells lining your blood vessels use to manufacture nitric oxide. A review of watermelon’s nutritional profile confirmed that L-citrulline found in high concentrations in watermelon may serve as a recycling substrate, increasing L-arginine availability and, consequently, nitric oxide synthesis.1PubMed Central. Watermelon Nutritional Composition with a Focus on L-Citrulline and Its Cardioprotective Health Effects-A Narrative Review
You might wonder why people don’t just take L-arginine directly. The reason is that L-arginine gets chewed up by enzymes in the gut and liver before most of it ever reaches your bloodstream. L-citrulline sidesteps that problem. It passes through the intestinal wall largely intact and then gets converted to L-arginine later, in the kidneys, where the arginine enters the bloodstream in a more useful form.2PubMed Central. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism A pharmacokinetic study found that oral L-citrulline raised blood levels of L-arginine more effectively than the same dose of L-arginine itself.3PubMed. Endogenous flux of nitric oxide: Citrulline is preferred to Arginine This is why most of the supplement research on erectile function has focused on citrulline rather than arginine.
What the Human Trials Found
The most-cited study on citrulline and erections is a small Italian trial from 2011. Twenty-four men with mild ED took 1.5 grams of L-citrulline daily for one month, then switched to placebo for another month (or vice versa). Half the men moved from a “mild ED” erection hardness score to a “normal” score while taking citrulline, compared to just two men (about 8 percent) while on placebo. The men also reported roughly doubling their monthly frequency of intercourse during the citrulline phase. No side effects were reported.4PubMed. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction
Those results are encouraging, but 24 participants is tiny by clinical-trial standards. A single small study can easily overestimate or underestimate an effect. Larger trials are needed before anyone can say with confidence how well citrulline works for ED in the general population.
A second pilot study tested a combination of L-citrulline and transresveratrol (a compound found in grape skins) in men who were already taking PDE5 inhibitor drugs but not getting satisfactory results. The combination improved self-reported erectile function scores compared to both baseline and placebo.5PubMed Central. Oral L-citrulline and Transresveratrol Supplementation Improves Erectile Function in Men With Phosphodiesterase 5 Inhibitors: A Randomized, Double-Blind, Placebo-Controlled Crossover Pilot Study This suggests citrulline might add something on top of standard medication for men with harder-to-treat cases, though again the study was small and involved a combination supplement rather than citrulline alone.
There is also indirect human evidence. Researchers measured blood levels of L-arginine and L-citrulline in men with different types and severities of ED. Men with arteriogenic ED (where restricted blood flow to the penis is the root cause) had significantly lower levels of both amino acids compared to healthy controls. And men with the most severe erectile dysfunction had roughly 13 to 17 percent lower levels than those with mild ED.6PubMed Central. Levels of l-arginine and l-citrulline in patients with erectile dysfunction of different etiology This doesn’t prove that raising those levels fixes the problem, but it does suggest that low citrulline and arginine availability is part of the picture for at least some men with ED.
How Much Watermelon Would You Actually Need
Here is where the “watermelon as Viagra” narrative runs into practical trouble. The clinical trial that showed benefit used 1.5 grams of pure L-citrulline per day. Watermelon flesh contains roughly 1.5 to 2 grams of citrulline per kilogram on a fresh-weight basis.7PubMed. Determination of citrulline in watermelon rind That means you would need to eat somewhere in the range of one to two pounds of watermelon flesh daily just to approach the study dose. That is not impossible, but it is a lot of fruit to commit to every single day, and the citrulline content varies by variety.
Interestingly, the rind contains more citrulline than the flesh on a dry-weight basis (though the difference narrows when you account for the flesh’s higher water content). Flesh color matters too: yellow and orange varieties tend to have higher citrulline concentrations than the common red-fleshed types.7PubMed. Determination of citrulline in watermelon rind Citrulline levels also vary depending on the growing conditions and specific cultivar. One study across different watermelon lines found the highest-citrulline genotype contained roughly 70 percent more citrulline than the lowest.8Journal of Agricultural Science and Technology. Citrulline Content of Fruit Flesh and Rind Parts in Different Watermelon Lines
None of this means eating watermelon is useless. It does contribute citrulline to your diet, and if you already enjoy the fruit, there is no harm in viewing it as a modest contributor to nitric oxide production. But if your goal is to reach the doses that showed benefit in clinical trials, a citrulline supplement (usually sold as citrulline malate in powder or capsule form) is a far more practical route.
Animal Studies and a Possible Second Mechanism
Rat studies add another layer of plausibility. In one experiment, rats whose erectile function was impaired by restricted blood flow to the pelvis showed significant improvement after oral L-citrulline supplementation. Their erectile pressures rose, their nitric oxide levels increased, and the ratio of smooth muscle to collagen in their penile tissue improved, suggesting the tissue itself was healthier.9PubMed. Oral L-citrulline supplementation improves erectile function in rats with acute arteriogenic erectile dysfunction A separate study in castrated rats (modeling low-testosterone-related ED) found similar structural and functional improvements with citrulline.10PubMed. Oral L-citrulline supplementation improves erectile function and penile structure in castrated rats Animal results don’t always translate to humans, but they do show that the mechanism works across more than one model of erectile dysfunction.
A computational biochemistry study recently proposed that citrulline might not only boost nitric oxide through the arginine pathway but also directly inhibit PDE5, the same enzyme that sildenafil blocks. The researchers’ molecular modeling suggested citrulline binds to PDE5 in a non-competitive manner, with high structural stability.11Scientific African. Mechanisms of L-citrulline on phosphodiesterase 5 in erectile dysfunction intervention This is early-stage work and has not been confirmed in living organisms, so it remains speculative. But if citrulline does have a dual action, boosting nitric oxide production while also slowing its downstream breakdown, that would make its potential more interesting than the arginine pathway alone would suggest.
The Blood Pressure Overlap
Erectile dysfunction and cardiovascular disease share the same underlying issue: blood vessels that don’t dilate the way they should. The evidence that watermelon and citrulline improve blood vessel function is actually stronger and more extensive than the specific ED evidence, and it matters for men dealing with erection problems because vascular health and erectile health are deeply intertwined.
A pilot study in people with prehypertension found that six weeks of watermelon supplementation significantly lowered central aortic blood pressure measures compared to placebo.12American Journal of Hypertension. Effects of Watermelon Supplementation on Aortic Blood Pressure and Wave Reflection in Individuals With Prehypertension: A Pilot Study In obese adults with prehypertension or stage 1 hypertension, watermelon extract reduced ankle systolic blood pressure by about 11.5 mmHg and brachial systolic by about 15 mmHg compared to placebo.13American Journal of Hypertension. Watermelon extract supplementation reduces ankle blood pressure and carotid augmentation index in obese adults with prehypertension or hypertension A recent meta-analysis of randomized controlled trials concluded that L-citrulline supplementation significantly improved flow-mediated dilation, a measure of how well arteries expand in response to blood flow.14PubMed Central. Effects of L-citrulline supplementation and watermelon intake on arterial stiffness and endothelial function in middle-aged and older adults: a systematic review and meta-analysis of randomized controlled trials
Not every vascular study has been positive. A trial in postmenopausal women who drank watermelon juice daily for several weeks found no changes in circulating arginine, citrulline, or any vascular measures, despite a bump in lycopene (another compound in watermelon).15PubMed Central. Daily 100% watermelon juice consumption and vascular function among postmenopausal women: A randomized controlled trial This is a reminder that whole-food doses may not always be high enough to move the needle, and that individual responses vary. Still, the overall direction of the vascular evidence supports the idea that citrulline has real effects on blood vessel relaxation, which is the same fundamental process that erections require.
The Placebo Effect Is Surprisingly Strong in ED
Any conversation about ED treatments needs to account for how powerful expectation alone can be. A large analysis pooling data from ED drug trials found that men assigned to placebo still experienced a small to moderate improvement in erectile function, with an effect size of about 0.35.16PubMed Central. Placebo Responses Among Men With Erectile Dysfunction Enrolled in Phosphodiesterase 5 Inhibitor Trials The effect was even bigger among men whose ED was associated with psychological factors like post-traumatic stress disorder.
This does not mean that citrulline’s benefits are purely placebo. The Italian trial’s 50 percent improvement rate in the citrulline group was much higher than the 8 percent in the placebo group, and the gap was statistically significant. But it does mean that when men read enthusiastic claims about watermelon and ED and then try it themselves, some of the benefit they feel may come from anticipation rather than biochemistry. In a sense, that is fine from the perspective of the man experiencing the improvement. But it makes it harder to know exactly how large the pharmacological effect is without much bigger and better-controlled trials.
Combining Citrulline With ED Medications
Some men wonder whether adding citrulline or arginine to their existing ED medication could boost results. The pilot study mentioned earlier, which combined citrulline with transresveratrol in men already on PDE5 inhibitors, did show improved erectile function scores.5PubMed Central. Oral L-citrulline and Transresveratrol Supplementation Improves Erectile Function in Men With Phosphodiesterase 5 Inhibitors: A Randomized, Double-Blind, Placebo-Controlled Crossover Pilot Study A separate trial tested the combination of sildenafil with L-arginine (the downstream product of citrulline) and found slightly better erectile function scores in the combination group compared to sildenafil alone, though the difference was modest and side effects like gastritis were a bit more common in the combination arm.17PubMed. Efficacy and tolerability of sildenafil/l-arginine combination relative to sildenafil alone in patients with organic erectile dysfunction
The logic behind the combination approach makes sense on paper: PDE5 inhibitors prevent the breakdown of the signal that nitric oxide triggers, while citrulline boosts nitric oxide production upstream. Feed more raw material in while slowing its clearance, and you might get a stronger response. The early evidence points in that direction, but it is not strong enough to be considered a standard recommendation. If you are taking prescription ED medication and want to add citrulline, it is worth discussing with a doctor, particularly because both citrulline and PDE5 inhibitors can lower blood pressure, and the combination could potentially cause dizziness or lightheadedness in some people.
Safety at Supplement Doses
L-citrulline supplements have a fairly clean safety record at the doses studied. The Italian ED trial used 1.5 grams daily with no reported side effects. Exercise and cardiovascular studies have used doses of 3 to 8 grams daily without serious adverse events. Citrulline in a study of young adults lowered blood pressure by about 6 percent systolic and 14 percent diastolic, effects that were larger than those from nitrate supplementation in the same study.18SpringerOpen. Oral nitrate and citrulline decrease blood pressure and increase vascular conductance in young adults: a potential therapy for heart failure For most people, a mild blood pressure reduction is actually a benefit. But if you already have low blood pressure, take blood pressure medication, or are using PDE5 inhibitors, the additive pressure drop is something to be aware of.
Gastrointestinal discomfort is the most common complaint at higher doses, generally above 6 grams. Citrulline can also interact with medications for angina or pulmonary hypertension that work through the nitric oxide pathway. If you are on any prescription medication that affects blood pressure or blood vessel dilation, talk to your doctor before starting a citrulline supplement.
Who Might Benefit Most
Based on the available evidence, citrulline supplementation is most likely to help men whose ED is mild and primarily vascular in origin. The clinical trial that showed benefit specifically enrolled men with mild ED, and the blood-level study found that men with arteriogenic ED had the lowest citrulline and arginine levels.6PubMed Central. Levels of l-arginine and l-citrulline in patients with erectile dysfunction of different etiology Men whose ED stems from nerve damage, severe hormonal deficiency, or psychological factors may see less benefit from a strategy that works primarily by improving blood vessel relaxation.
It is also worth recognizing that ED is often an early warning sign of broader cardiovascular problems. A man whose erections have gotten weaker may have developing atherosclerosis affecting not just the penile arteries but also the coronary arteries. In that context, citrulline’s broader vascular benefits, including improved arterial flexibility and modest blood pressure reductions, could matter beyond the bedroom. But those same cardiovascular concerns are also a reason to see a doctor rather than relying entirely on a dietary supplement. ED medication is well-studied and effective; watermelon and citrulline supplements are not substitutes for it when the underlying problem is significant vascular disease.
Why Red Versus Yellow Watermelon Matters
If you are interested in getting more citrulline from actual watermelon rather than a supplement, variety matters more than most people realize. The common red-fleshed watermelons that dominate grocery stores tend to have lower citrulline concentrations than yellow or orange varieties. In one analysis, yellow-fleshed watermelons had nearly four times the citrulline content of red-fleshed ones on a dry-weight basis.7PubMed. Determination of citrulline in watermelon rind Skin color of the rind also appears to correlate with citrulline levels, with lighter-skinned genotypes producing more.8Journal of Agricultural Science and Technology. Citrulline Content of Fruit Flesh and Rind Parts in Different Watermelon Lines
The rind, which most people discard, is also relatively citrulline-rich. Some cultures pickle or stir-fry watermelon rind, and a few supplement companies have started producing watermelon rind extracts. Whether eating rind regularly in meaningful quantities is realistic for most people is another question entirely, but if you are already the type to minimize food waste, blending some of the white rind into a smoothie is one way to bump up your citrulline intake without a supplement bottle.