Nitric oxide is a signaling molecule your body produces naturally that widens blood vessels, helps deliver oxygen and nutrients to working muscles, and plays a role in how quickly your muscles generate force. It’s one of the most marketed molecules in the supplement industry, but its actual effects during exercise are more nuanced than most pre-workout labels suggest.
How Nitric Oxide Affects Blood Flow
Nitric oxide relaxes the smooth muscle lining your blood vessels, causing them to dilate. Wider vessels mean more blood can reach your muscles during a set or a run. In theory, this delivers more oxygen and clears metabolic waste faster, letting you train harder and recover between efforts more quickly.
The real-world picture is a bit more modest. A review in the Canadian Journal of Applied Physiology examined nitric oxide’s role in muscle blood flow during exercise and found its contribution to steady-state exercise blood flow, the moment-to-moment adjustment of flow during movement, and the overriding of stress-related vessel constriction was “modest at best.” Your body relies on several overlapping systems to push blood into working muscles, and nitric oxide is just one piece of that puzzle. It matters, but it isn’t the master switch that supplement marketing often implies.
Effects on Force and Power Output
Where nitric oxide shows more interesting effects is in muscle contraction itself. Research published in PMC found that boosting nitric oxide through dietary means accelerated the rate of force development in both mouse and human muscle, without changing the maximum force those muscles could produce. In practical terms, this means your muscles may contract faster, not necessarily harder. For movements that depend on speed and explosiveness (think jumping, sprinting, or the initial pull of a deadlift), a faster rate of force development can translate into more power.
Peak power and unloaded shortening velocity, which is essentially how fast a muscle fiber can shorten when it isn’t fighting heavy resistance, also improved with nitric oxide manipulation. The current hypothesis is that nitric oxide modifies proteins inside the muscle fiber that regulate how cross-bridges (the tiny engines inside muscle cells) cycle during contraction. Maximal isometric force, the absolute ceiling of what a muscle can produce, stays the same at normal nitric oxide levels. So think of the benefit as making your existing strength more available more quickly, rather than making you fundamentally stronger.
Nitric Oxide and Muscle Soreness
Nitric oxide has a complicated relationship with delayed-onset muscle soreness (DOMS), that deep ache you feel one to three days after a tough workout. Your muscle tissue produces nitric oxide through several different enzyme pathways, and after hard or unfamiliar exercise, those pathways become more active.
On one hand, nitric oxide activates satellite cells, the repair crew that fuses with damaged muscle fibers and helps rebuild them. It also stimulates follistatin, a protein involved in muscle repair. On the other hand, nitric oxide can trigger pain-sensing nerve fibers (C-fibers) in muscle tissue, contributing to the soreness itself. Research from ScienceDirect highlights that nitric oxide likely plays a role in both the decreased force output you experience during DOMS and the pain sensation that accompanies it. Blocking nitric oxide could theoretically reduce soreness, but it might also slow down the repair process. DOMS is too complex to pin on any single molecule, but nitric oxide appears to be involved on both sides of the equation.
Citrulline vs. Arginine for Boosting Levels
Your body makes nitric oxide from the amino acid arginine. For years, arginine supplements were the go-to recommendation for anyone wanting to boost nitric oxide before training. That recommendation has largely shifted to citrulline, and the reason comes down to how your gut and liver process each one.
When you swallow arginine, roughly 70% of it gets extracted by your gut and liver before it ever reaches general circulation. Only about 30% makes it into your bloodstream, and even that amount fails to meaningfully increase total arginine turnover in the body. Citrulline, by contrast, passes through the gut and liver almost entirely intact (around 105% recovery in plasma, accounting for measurement variability). Your kidneys then convert citrulline into arginine, effectively bypassing the bottleneck. In a head-to-head comparison published in The Journal of Nutrition, citrulline supplementation raised plasma arginine concentrations 35% more than arginine supplementation itself. That higher arginine availability is what drives greater nitric oxide production.
Dosing and Timing
Citrulline, often sold as citrulline malate (citrulline bonded to malic acid), is the most common nitric oxide booster in pre-workout supplements. About 70% of popular pre-workout formulas contain it, though the amount varies widely. Many commercial blends average around 4 grams, but studies examining performance benefits most commonly use 6 to 8 grams taken 40 to 120 minutes before exercise. Doses as low as 3 grams have been validated in team-sport athletes, but the stronger body of evidence clusters around that 6 to 8 gram range for a single pre-exercise dose.
If your pre-workout supplement lists citrulline in a “proprietary blend” without specifying the amount, you likely aren’t getting enough to match what’s been studied. Standalone citrulline malate powder is inexpensive and lets you control the dose precisely.
Who Should Be Cautious
Because nitric oxide lowers blood pressure by widening blood vessels, supplements that boost it can compound the effects of blood pressure medications or diabetes medications. People with already low blood pressure, liver cirrhosis, or a rare metabolic condition called guanidinoacetate methyltransferase deficiency face a higher risk of side effects. If you take blood pressure or blood sugar medication, the interaction is worth discussing before adding a nitric oxide booster to your routine. Anyone with low blood pressure should also stop supplementation before any scheduled surgery, since the combination with anesthesia can cause dangerous drops in blood pressure.
For most healthy people, citrulline at studied doses is well tolerated. The most commonly reported side effect is mild gastrointestinal discomfort, which tends to occur at higher doses taken on an empty stomach.