Nitric oxide does affect testosterone production, but the relationship is far more complicated than supplement marketing suggests. Depending on where in the body it acts and how much is present, nitric oxide can either raise or lower testosterone. At the level of the brain, it helps trigger the hormonal cascade that tells the testes to produce testosterone. Inside the testes themselves, a small amount of nitric oxide appears to stimulate testosterone production, while a large amount shuts it down. This dose-dependent, location-dependent duality means that simply “boosting NO” does not translate neatly into higher testosterone.
The Brain Side of the Equation
Testosterone production begins with a signal from the brain. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which travels to the pituitary gland and prompts it to release luteinizing hormone (LH). LH then tells the Leydig cells in the testes to make testosterone. Nitric oxide plays a role right at the start of this chain. Research in rats has shown that NO stimulates both the release and the gene expression of GnRH in the hypothalamus. When researchers blocked NO production in the brain, GnRH messenger RNA dropped by roughly 20%, and this effect was even more pronounced in castrated animals, where blocking NO cut GnRH expression by about 39%.1Brain Research. Role of nitric oxide in the regulation of gonadotropin-releasing hormone and tyrosine hydroxylase gene expression in the male rat brain
The mechanism involves NO activating an enzyme called guanylate cyclase, which raises levels of a signaling molecule (cGMP) that in turn triggers GnRH release.2Biochemistry and Cell Biology. The role of glutamate and nitric oxide in the reproductive neuroendocrine system So at the hypothalamic level, NO is genuinely pro-testosterone. It helps initiate the whole hormonal cascade. This is one reason why poor vascular health or chronic conditions that reduce NO availability can coincide with lower testosterone: the brain’s signaling to the testes may be weaker.
What Happens Inside the Testes
If nitric oxide only acted in the brain, the story would be straightforward. But NO is also produced locally inside the testes, by the same Leydig cells responsible for making testosterone as well as by nearby immune cells and blood vessel walls.3PubMed Central. Evidence for production and functional activity of nitric oxide in seminiferous tubules and blood vessels of the human testis And at this local level, the picture flips.
In cell studies, exposing Leydig cells to NO donors inhibited testosterone production. The NO appeared to directly interfere with the enzyme that converts cholesterol into pregnenolone, the very first step in building a testosterone molecule. This enzyme contains an iron-containing structure that NO can bind to, essentially jamming the machinery.4PubMed. Nitric oxide inhibits Leydig cell steroidogenesis In a separate experiment, when researchers blocked NO production inside testicular tissue using specific inhibitors, both basal and hormone-stimulated testosterone production went up, while the signaling molecule cGMP dropped. That suggests that under normal conditions, NO inside the testes acts as a local brake on testosterone output.5Metabolism. Evidence to suggest nitric oxide is an interstitial regulator of Leydig cell steroidogenesis
So the brain says “more NO helps testosterone” while the testes themselves say “more NO slows testosterone down.” This is not actually contradictory when you consider the different concentrations and contexts involved, which brings us to one of the most clarifying findings in this area.
Low Dose Versus High Dose
A key study on purified rat Leydig cells tested three different NO donors across a wide range of concentrations. At low concentrations, NO increased both testosterone and cGMP secretion. At higher concentrations, testosterone production was inhibited, though cGMP was not affected. When the researchers added a substance that scavenges NO, both the stimulatory and inhibitory effects disappeared, confirming that NO itself was responsible for both outcomes.6PubMed. Biphasic effect of nitric oxide on testosterone and cyclic GMP production by purified rat Leydig cells cultured in vitro
This biphasic pattern explains a lot of the seemingly contradictory research. At physiologically normal, low levels, nitric oxide gently supports testosterone production in the testes, likely through the cGMP pathway. When NO floods in at high concentrations, as happens during severe inflammation or with very high doses of NO donors, it directly poisons the enzyme responsible for the first step of steroid synthesis. The practical implication is that more NO is not automatically better for testosterone. There is a sweet spot, and overshooting it can backfire.
When Inflammation Sends NO Through the Roof
The body produces nitric oxide through different enzymes. Two of them, endothelial NOS (eNOS) and neuronal NOS (nNOS), generate modest, controlled amounts of NO for everyday functions like blood vessel relaxation and neurotransmission. A third, inducible NOS (iNOS), ramps up production dramatically in response to infection, injury, or chronic inflammation, sometimes producing hundreds of times more NO than the other two.
This distinction matters because testicular inflammation can cause iNOS levels to skyrocket. In rat models, conditions like diabetes combined with alcohol exposure and antiretroviral therapy led to significantly elevated iNOS expression in the testes compared to healthy controls.7PubMed Central. Potential role of inducible nitric oxide synthase (iNOS) activity in testicular dysfunction following co-administration of alcohol and combination antiretroviral therapy (cART) in diabetic rats: an immunohistochemistry study Research has confirmed that NO can play a role in suppressing testosterone production and disrupting sperm production during testicular inflammation.8Biology of Reproduction. Inducible Nitric Oxide Synthase in the Rat Testis: Evidence for Potential Roles in Both Normal Function and Inflammation-Mediated Infertility
This is the ugly version of the NO-testosterone interaction. Chronic infections, autoimmune conditions, obesity-related inflammation, and other states that activate iNOS in or near the testes can dump enough NO to suppress testosterone. Paradoxically, someone with chronic inflammation might benefit from reducing NO at the testicular level while maintaining healthy NO signaling in the brain and blood vessels. The body’s compartmentalization of NO production makes blanket “boost your NO” advice misleading for anyone dealing with an inflammatory condition.
Stress, Cortisol, and a Common Misconception
A popular claim in wellness circles is that NO suppresses cortisol, which then raises testosterone indirectly. There is a kernel of truth here. In fetal adrenal cells from sheep, pretreating with NO donors or the NO precursor L-arginine abolished the elevated cortisol response to hormonal stimulation.9PubMed Central. Nitric Oxide Inhibits ACTH-Induced Cortisol Production in Near Term, Long Term Hypoxic Ovine Fetal Adrenocortical Cells But this was in fetal sheep cells under oxygen deprivation, a specialized scenario far removed from a healthy adult human taking a supplement.
When researchers tested this more directly by looking at mice genetically engineered to lack the iNOS enzyme, they found that stress still crushed testosterone levels in these knockout animals just as thoroughly as in normal mice. The stable byproducts of NO in testicular tissue did not differ between stressed and unstressed animals regardless of genotype. The researchers concluded that cortisol, not NO, was the plausible mediator of rapid stress-induced testosterone suppression.10American Journal of Physiology-Endocrinology and Metabolism. Testosterone production in mice lacking inducible nitric oxide synthase expression is sensitive to restraint stress So while NO can theoretically dampen cortisol production in narrow experimental conditions, the evidence for this being a meaningful testosterone-boosting pathway in real life is weak.
What About L-Arginine and L-Citrulline Supplements?
L-arginine is the amino acid the body uses to manufacture nitric oxide, and L-citrulline is recycled back into L-arginine after NO is produced. Both are widely sold as NO-boosting supplements. Do they raise testosterone?
In men with type 2 diabetes, a double-blind trial found that eight weeks of L-arginine supplementation led to a significantly greater increase in testosterone compared to placebo, with the supplement group gaining roughly four times the testosterone increase seen in the placebo group.11Journal of Nutrition and Food Security. Effect of Oral Supplementation of L-arginine on Sexual Function in Men with Type 2 Diabetes: A Double-blind Clinical Trial A separate trial in diabetic men likewise found that L-arginine supplementation raised free testosterone levels over eight weeks, while the placebo group saw no meaningful change.12Journal of Isfahan Medical School. Effect of L-Arginine Supplementation on Sexual Function and Hypogonadism in Men with Type 2 Diabetes
These are encouraging results, but they come with major caveats. Both studies were in men with type 2 diabetes, a condition characterized by vascular dysfunction, impaired NO synthesis, and often low testosterone. L-arginine may have restored NO levels back toward normal rather than pushing them above baseline, which would be consistent with the biphasic model: moving from deficiency to adequacy helped, but that does not mean loading more NO on top of healthy levels would do the same. No large trial has tested L-arginine’s effect on testosterone in healthy men with normal hormonal function.
For L-citrulline, the human data on testosterone is essentially nonexistent. The available studies are in animals. In male camels under heat stress, intravenous L-citrulline raised plasma testosterone beginning about four hours after injection and lasting up to 72 hours, alongside increases in NO levels.13PubMed. L-Citrulline Single Intravenous Administration Promotes Testicular Blood Flow, Plasma Testosterone, Estradiol, Circulating Citrulline and Serum Nitric Oxide Levels in Male Camels Under Heat Stress at the End of Non-Breeding Season In rams, oral L-citrulline supplementation increased GnRH, LH, FSH, and testosterone alongside NO levels in both blood and seminal plasma.14PubMed. Effect of L-citrulline supplementation on sperm characteristics and hormonal and antioxidant levels in blood and seminal plasma of rams And in dogs, L-citrulline combined with sildenafil raised testosterone levels initially, though they fell back down within 48 hours.15PubMed. The impacts of L-citrulline and sildenafil on penile and testicular vascular perfusion pattern, advanced testicular analysis, steroids, and semen picture in normospermic dogs
These animal findings are consistent with the idea that boosting NO through its precursors can enhance testicular blood flow and support the hormonal chain from hypothalamus to testes. But extrapolating from camels and rams to men taking a scoop of citrulline malate before a workout is a stretch. The doses, delivery methods, and species physiology are all different. At present, L-citrulline’s testosterone effects in humans remain untested in any published trial.
PDE5 Inhibitors and an Unexpected Testosterone Effect
PDE5 inhibitors like sildenafil (Viagra) and tadalafil (Cialis) work by blocking the enzyme that breaks down cGMP, the molecule that NO activates to relax blood vessels. They effectively amplify whatever NO signaling is already present. Given the discussion above, you might wonder whether extending NO’s downstream effects also influences testosterone. The answer appears to be yes, at least sometimes.
A study in men with metabolic syndrome found that three months of daily low-dose tadalafil raised mean testosterone from about 3.6 to 5.2 (in their measurement units), while LH levels dropped from 5.6 to 4.6, suggesting the testosterone increase was happening at the testicular level rather than being driven by the brain sending stronger signals.16PubMed. Effects of taking tadalafil 5 mg once daily on erectile function and total testosterone levels in patients with metabolic syndrome A study with sildenafil found even more striking results: total testosterone rose by about 100 ng/dL on average, and serum LH actually decreased, pointing to a direct stimulatory effect on the testes rather than increased brain signaling.17PubMed Central. Sildenafil increases serum testosterone levels by a direct action on the testes
However, an earlier pilot study of long-term tadalafil use found no significant changes in total or free testosterone.18PubMed. Testosterone:estradiol ratio changes associated with long-term tadalafil administration: a pilot study The discrepancy may come down to the patient populations involved. Men with metabolic syndrome often have impaired NO signaling and lower baseline testosterone, so restoring effective NO activity might produce a noticeable hormonal bump. In men who already have healthy vascular function, amplifying NO’s downstream effects may not translate to measurably higher testosterone. Again, the pattern echoes the biphasic model: pushing from deficiency toward adequacy helps; piling more on top of normal may not.
Aging, Vascular Health, and the Shared Decline
As men age, both NO production and testosterone tend to decline in parallel. Aging reduces the expression and activity of endothelial NOS, which means less NO available for blood vessel relaxation, and simultaneously brings changes in arterial structure and circulating hormone levels.19PubMed. Endothelial dysfunction and erectile dysfunction in the aging man This parallel decline has led some to speculate that falling NO is partly responsible for falling testosterone in older men, rather than both being independent consequences of aging.
There is biological plausibility to this idea. NO is produced inside testicular blood vessels and helps regulate testicular blood flow. The testes depend on adequate blood supply to receive LH signals and deliver raw materials for steroid synthesis. If aging arteries produce less NO and deliver less blood to the testes, Leydig cell function could suffer even if the cells themselves are still capable of producing testosterone. The supplement and PDE5 inhibitor data are somewhat consistent with this theory: men whose NO signaling was compromised (by diabetes, metabolic syndrome, or aging) seemed to get a testosterone benefit from interventions that restored it. Whether maintaining healthy NO levels throughout life could meaningfully slow age-related testosterone decline is an appealing hypothesis that remains untested in long-term human trials.
Safety Considerations for NO-Boosting Supplements
NO-boosting supplements, primarily L-arginine and L-citrulline, are generally well tolerated at standard doses but can cause gastrointestinal discomfort, bloating, and diarrhea. Using smaller or divided doses can reduce these side effects. More seriously, these supplements can interact with blood pressure medications and blood thinners, and they may worsen certain conditions. Medical supervision is recommended, particularly for anyone already on medication for cardiovascular issues.20PubMed Central. Dietary supplements for improving nitric-oxide synthesis
It is also worth noting that the commercial “testosterone booster” market is poorly regulated. One case report examining a commercial testosterone-boosting product found that while it did produce a slight testosterone increase after four courses, it also negatively affected several liver function markers.21PubMed Central. Effect of testosterone boosters on body functions: Case report Many products combine NO precursors with a grab bag of other ingredients, making it impossible to attribute effects (or side effects) to any single component. If you are considering supplements specifically to influence testosterone through the NO pathway, isolated L-arginine or L-citrulline in known doses is a far more interpretable choice than a proprietary blend with a muscular label.
NO as an Endocrine Signal
For decades, nitric oxide was thought to act only locally, as a very short-range signal that diffused a few cells away and then disappeared. That understanding has shifted. Researchers now recognize that NO can function as an endocrine molecule, exerting effects at sites distant from where it was produced.22PubMed. Nitric oxide: To be or not to be an endocrine hormone? This reclassification matters because it means NO produced in the vascular endothelium, in the brain, or even in the gut may influence testicular function in ways that were not previously considered possible. The field is still working out how much distance-signaling NO actually contributes to hormonal regulation versus how much is handled by local production within the testes. But the fact that NO travels farther than once assumed opens the door to the idea that systemic vascular health, dietary nitrate intake, and exercise habits could all influence testicular steroidogenesis through NO pathways that have nothing to do with supplements.