L-citrulline is in your pre-workout to boost nitric oxide production, which widens blood vessels and increases blood flow to working muscles. This can translate into more reps on later sets, reduced muscle soreness after training, and a stronger “pump.” It’s one of the more evidence-backed ingredients on a typical pre-workout label, though the benefits depend heavily on getting the right dose.
How L-Citrulline Becomes Nitric Oxide
L-citrulline is an amino acid that your body converts into another amino acid, L-arginine, through a two-step enzymatic process. L-arginine then serves as the raw material for producing nitric oxide, a molecule that signals blood vessels to relax and expand. The widened vessels allow more oxygen and nutrient-rich blood to reach your muscles during exercise.
You might wonder why pre-workouts don’t just use L-arginine directly. The reason is that L-arginine taken orally gets heavily broken down in the gut and liver before it ever reaches your bloodstream. L-citrulline bypasses that breakdown, gets absorbed intact, and then converts to L-arginine in the kidneys and other tissues. It’s a more efficient way to raise arginine levels and, ultimately, nitric oxide.
There’s also a recycling loop at work. When nitric oxide is produced, L-citrulline is generated as a byproduct. Your body then feeds that citrulline back through the same pathway to make more arginine and more nitric oxide. Supplementing with extra citrulline keeps this cycle running at a higher rate.
More Reps on Later Sets
The primary performance benefit of L-citrulline shows up in training volume, specifically the number of reps you can complete before failure. The effect tends to appear in the later sets of an exercise, when fatigue is highest. In trials using 8 grams of citrulline malate taken one hour before training, subjects completed more reps on exercises like bench press, leg press, hack squat, chin-ups, and push-ups compared to placebo. One study found subjects averaged about 32 chin-ups with citrulline malate versus 28 with placebo. On leg press, another trial showed roughly 67 reps versus 55.
These aren’t massive jumps per set, but they add up across an entire workout. If you’re consistently getting one or two extra reps on the back half of your session, that compounds into meaningful additional training volume over weeks and months. The gains are most noticeable during high-rep, strength-endurance style training rather than low-rep maximal strength work. Trials measuring one-rep max improvements over eight weeks have generally not found a significant effect.
It’s worth noting that the research is mixed overall. Some well-designed studies have found no benefit to a single dose on repetition performance. The positive results cluster around protocols with multiple sets taken close to failure, where fatigue management matters most.
Reduced Soreness After Training
One of the more striking findings is citrulline’s effect on post-workout muscle soreness. In a study on pectoral training, subjects who took citrulline malate reported 40% less muscle soreness at both 24 and 48 hours after their session, with over 90% of participants experiencing some degree of relief. The likely mechanism ties back to improved blood flow: better circulation during and after exercise helps clear metabolic waste products like ammonia that contribute to fatigue and soreness. Animal research has shown citrulline can reduce ammonia accumulation by a significant margin during prolonged exercise.
Modest Benefits for Cardio
L-citrulline’s effects on aerobic exercise are less dramatic than its effects on resistance training, but they exist. Supplementation has been shown to improve how quickly your body ramps up oxygen usage at the start of exercise, particularly in men. In one study, men who took citrulline reached their steady-state oxygen consumption about 4.5 seconds faster than with placebo. That might sound small, but a faster oxygen ramp-up means your body relies less on anaerobic energy at the start of a workout, which can preserve stamina for later.
Interestingly, this effect was not observed in women in the same study, and the reasons aren’t fully understood. The impact on older adults was also minimal. For cardio-focused athletes, citrulline is a secondary supplement at best, behind well-established options like caffeine or beetroot juice.
L-Citrulline vs. Citrulline Malate
You’ll see two forms on supplement labels: pure L-citrulline and citrulline malate (L-citrulline bonded to malic acid). Most of the positive resistance training studies used citrulline malate at 8 grams. The malic acid component plays a role in the energy production cycle inside your cells, and one study found a 34% increase in aerobic energy production after 15 days of 6 grams per day of citrulline malate.
The practical difference matters for dosing. An 8-gram dose of citrulline malate (typically in a 2:1 ratio) delivers roughly 5.3 grams of actual L-citrulline and 2.7 grams of malic acid. If your pre-workout uses pure L-citrulline, you need about 6 grams to match the citrulline content of the studied citrulline malate doses. Many pre-workouts contain only 2 to 3 grams, which is likely underdosed based on the research.
Dosing and Timing
The effective dose in clinical trials is 8 grams of citrulline malate or 6 grams of pure L-citrulline, taken as a single dose about 60 minutes before training. Nearly every study showing a positive effect on repetition performance used this protocol. Check your pre-workout label carefully. If the citrulline is buried inside a “proprietary blend” or listed at 1 to 3 grams, you’re probably not getting enough to see the benefits the research describes.
Some evidence also supports daily use. A 15-day loading protocol at 6 grams per day of citrulline malate improved aerobic energy production, and a meta-analysis on blood pressure found that doses of 6 grams or more per day produced a small but significant reduction in diastolic blood pressure (about 2.75 mmHg). This suggests citrulline’s vascular benefits may build with consistent use rather than relying solely on a pre-workout dose.
Side Effects
L-citrulline is well tolerated at the doses used in exercise research. Single doses in the 3 to 6 gram range rarely cause any side effects. Gastrointestinal discomfort, including nausea and diarrhea, becomes more likely at single doses above 9 grams, particularly in healthy, active people. If your pre-workout already contains 6 to 8 grams of citrulline malate and you’re stacking additional citrulline on top, that’s where you could run into stomach issues.
Because citrulline increases nitric oxide and can lower blood pressure slightly, people already taking blood pressure medication should be aware of the potential for an additive effect.
What to Look for on the Label
When evaluating a pre-workout for its citrulline content, three things matter: the form (L-citrulline or citrulline malate), the dose, and whether it’s inside a proprietary blend. You want at least 6 grams of pure L-citrulline or 8 grams of citrulline malate listed with an exact amount. If the label groups citrulline with other ingredients under a single weight, you have no way to know how much you’re actually getting. Many budget pre-workouts include citrulline at token doses, enough to put on the label but not enough to do anything meaningful.