L-citrulline does not increase testosterone. Human clinical trials measuring hormonal responses to citrulline supplementation have consistently found no change in testosterone levels compared to placebo. The supplement has real benefits for blood flow and exercise performance, but boosting testosterone isn’t one of them.
This matters because L-citrulline is widely marketed alongside testosterone-related claims, often because of its well-documented effects on erection quality. But those effects work through an entirely different mechanism, and conflating the two leads to confusion about what this supplement actually does in your body.
What the Human Trials Show
Two well-designed studies have directly measured testosterone responses to citrulline supplementation in young, active men. In one, 27 recreationally active men (average age 22) took an 8-gram dose two hours before resistance exercise. Researchers measured their hormonal response and found no difference from placebo. In the other, nine active men took 6 grams of citrulline malate one hour before leg press and hack squat sessions, with testosterone-to-cortisol ratios tracked at 24, 48, and 72 hours after exercise. Again, no change compared to placebo. Both studies used double-blind, randomized, placebo-controlled designs, which is the gold standard for supplement research.
The symbol researchers use in these papers is “↔,” meaning the values were statistically identical between the supplement and placebo groups. Not slightly higher, not trending upward. Flat.
Why Nitric Oxide Might Actually Suppress Testosterone
L-citrulline’s primary job in the body is to raise levels of arginine, which then gets converted into nitric oxide. This is the mechanism behind its cardiovascular and blood flow benefits. But here’s what makes the testosterone connection even less plausible: nitric oxide has been shown to suppress testosterone production, not enhance it.
Research from the American Physiological Society found that nitric oxide potently reduces testosterone production both in living animals and in isolated testicular cells. The cells responsible for making testosterone (Leydig cells) are directly inhibited by nitric oxide. At moderate concentrations, this molecule essentially shuts down the steroid-making machinery in those cells. One specific form of the enzyme that produces nitric oxide was directly implicated in this suppression, particularly during periods of stress when nitric oxide production ramps up.
This doesn’t mean taking citrulline will tank your testosterone. The nitric oxide increase from supplementation is modest and localized enough that it doesn’t appear to move testosterone levels in either direction based on the human data. But the underlying biochemistry runs counter to the idea that more nitric oxide equals more testosterone.
Where the Confusion Comes From
L-citrulline has a proven benefit for erection quality, and many people associate stronger erections with higher testosterone. A clinical trial of men with mild erectile dysfunction found that oral L-citrulline supplementation improved erection hardness. But the mechanism is entirely vascular, not hormonal. Citrulline converts to arginine, arginine produces nitric oxide, and nitric oxide relaxes blood vessels in the penis, allowing more blood flow. It’s the same pathway that medications for erectile dysfunction target.
Citrulline is actually better at raising arginine levels than arginine supplements themselves. When you swallow arginine directly, about 70% of it gets broken down in your gut and liver before reaching your bloodstream. Citrulline bypasses that breakdown almost entirely, with essentially 100% of supplemental citrulline appearing in plasma. In animal studies, the highest citrulline dose raised plasma arginine to 214 micromoles per liter, compared to 159 for the highest arginine dose and 109 for controls. So citrulline is a genuinely effective way to boost nitric oxide, which is genuinely effective for blood flow. It just has nothing to do with testosterone.
Animal Studies on Reproductive Health
Some animal research adds nuance but doesn’t change the picture for testosterone. One study in sheep found that L-citrulline supplementation increased reproductive hormone levels, antioxidant capacity, and semen quality in rams. Another found that citrulline protected testicular cells from DNA damage in sheep and mice fed unhealthy diets, improving sperm count and quality by supporting cellular repair mechanisms and the gut-testis connection.
These findings suggest citrulline may support reproductive health through antioxidant and cellular protection pathways, particularly under metabolic stress. But protecting testicular cells from damage is different from increasing testosterone output, and animal reproductive physiology doesn’t translate directly to humans. No human study has replicated these reproductive benefits.
Separately, a study in broiler chickens found that citrulline increased circulating growth hormone and insulin-like growth factor-1 concentrations. This is sometimes cited in marketing materials, but poultry metabolism bears little resemblance to human endocrine function, and no human trial has shown citrulline raising growth hormone levels.
What L-Citrulline Is Actually Good For
If you’re considering citrulline, it’s worth knowing what the evidence does support. Its strongest benefits are in cardiovascular function: improving blood flow, reducing blood pressure, and enhancing exercise endurance by helping your body clear metabolic waste products like ammonia and lactate during intense activity. The erection quality benefit is real and well-documented for mild cases. These are all nitric oxide-mediated effects that have nothing to do with hormone levels.
A safe daily dose is 3 to 6 grams of pure L-citrulline. Clinical studies have used doses up to 8 grams of citrulline malate (which is roughly 55% citrulline and 45% malic acid by weight). Starting at the lower end is reasonable since citrulline can cause bloating, cramping, or diarrhea in some people. Timing in studies is typically one to two hours before exercise.
Better Options for Testosterone Support
If your goal is specifically to support healthy testosterone levels, the evidence points elsewhere. Sleep quality, resistance training, maintaining a healthy body fat percentage, zinc and vitamin D sufficiency, and stress management all have stronger links to testosterone production than any single supplement. For men with clinically low testosterone, the effective interventions are medical, not supplemental.
L-citrulline is a useful supplement for the right reasons. Testosterone just isn’t one of them.