L-citrulline is an amino acid that your body converts into arginine, which then increases production of nitric oxide, a molecule that widens blood vessels and improves blood flow to working muscles. This process is why it’s one of the most common ingredients in pre-workout supplements. The performance benefits are real but modest: a meta-analysis of strength training studies found that citrulline malate increased total repetitions by about 6.4% compared to placebo.
How L-Citrulline Works in Your Body
When you swallow L-citrulline, your kidneys and blood vessels convert it into arginine through a two-step enzymatic process. That arginine then fuels production of nitric oxide, which relaxes blood vessel walls and increases blood flow. More blood flow means more oxygen and nutrients reaching your muscles during a workout, and faster removal of metabolic waste products like ammonia that contribute to fatigue.
Here’s what makes citrulline interesting: taking it actually raises your blood arginine levels more effectively than taking arginine itself. In animal research, citrulline supplementation increased plasma arginine concentrations by 35% more than an equivalent arginine supplement. The reason is that oral arginine gets heavily broken down in the gut and liver before it ever reaches your bloodstream, while citrulline bypasses that breakdown and gets converted to arginine later. Arginine supplements also failed to increase the rate of arginine circulating through the body, while citrulline succeeded.
Effects on Strength Training
The most studied use of citrulline is resistance training, specifically whether it lets you squeeze out more reps before failure. A systematic review and meta-analysis of strength training studies found that 6 to 8 grams of citrulline malate taken 40 to 60 minutes before exercise increased total repetitions by an average of 3 reps, or about 6.4%, compared to placebo. The effect was statistically significant but small in magnitude.
The benefits appeared slightly stronger for lower body exercises, where repetitions trended toward an 8.1% increase, compared to upper body exercises at 5.7% (which didn’t reach statistical significance). The practical takeaway: if you’re doing high-rep sets of squats or leg presses, you’re more likely to notice the effect than on bench press or rows. The researchers concluded that citrulline malate may delay fatigue during high-intensity strength training, allowing more total work across multiple sets, which could improve the training stimulus over time.
Not every study agrees, though. One study giving a single 8-gram dose of pure L-citrulline found no difference in bench press repetitions, isometric force production, or barbell velocity compared to placebo. The inconsistency across studies is worth noting: citrulline is not going to transform your workouts, but it may provide a small edge in muscular endurance.
Effects on Endurance Exercise
For cardio and endurance work, the picture is mixed and depends heavily on how long you’ve been taking it. A single dose before a treadmill test did not improve time to exhaustion or oxygen consumption. But when taken daily for 7 to 16 days, L-citrulline at doses of 2.4 to 6 grams per day improved several performance markers. In trained male cyclists, seven days of 6-gram daily dosing improved time trial performance by about 5.2% and average power output by 5.4%. A separate study found that 2.4 grams per day for eight days reduced time trial completion time by 1.5% and increased mean power output by 2%.
Chronic supplementation also appears to improve how efficiently your body uses oxygen. One early study found that 15 days of citrulline malate supplementation at 6 grams per day increased the contribution of aerobic energy production by 34%. Citrulline also helps reduce ammonia accumulation during exercise, which is one of the biochemical signals that drives the sensation of fatigue.
Recovery and Muscle Soreness
Citrulline shows a meaningful effect on next-day soreness. A meta-analysis of seven studies found that citrulline supplementation significantly reduced muscle soreness ratings at 24 hours post-exercise, with a large effect size. The same analysis found that soreness at 48 hours was also reduced, though the evidence was somewhat less consistent across studies. Citrulline also significantly reduced ratings of perceived exertion during exercise itself, meaning the same workout felt slightly easier.
The reduced soreness likely ties back to improved blood flow. Better circulation during and after exercise helps clear inflammatory byproducts and delivers nutrients needed for muscle repair more efficiently.
L-Citrulline vs. Citrulline Malate
You’ll find two forms on supplement shelves: pure L-citrulline and citrulline malate, which combines citrulline with malic acid (usually in a 2:1 ratio). Most of the resistance training research has used citrulline malate, particularly at an 8-gram dose. Malic acid plays a role in your cells’ energy production cycle, and one study found that citrulline malate increased aerobic energy production by 34% over 15 days of use. Whether the malic acid component adds meaningful benefit beyond the citrulline alone is still debated.
If you’re using citrulline malate, an 8-gram dose gives you roughly 5.3 grams of actual citrulline and 2.7 grams of malic acid (assuming a 2:1 ratio). If you’re using pure L-citrulline, 3 to 6 grams is the typical range studied for endurance benefits. For a fair comparison, you’d want to match the citrulline content, not the total gram weight.
Dosage and Timing
For resistance training, the most studied protocol is 6 to 8 grams of citrulline malate taken as a single dose about 40 to 60 minutes before your workout. For pure L-citrulline targeting endurance performance, daily doses of 2.4 to 6 grams taken consistently for at least seven days appear more effective than a single pre-workout dose. Dose-response research in healthy adults has tested single doses up to 15 grams of citrulline and found that larger doses produce higher plasma arginine levels, suggesting the commonly used amounts may actually be on the conservative side.
The acute-versus-chronic distinction matters. Single pre-workout doses have shown benefits for rep performance in some studies but not others. Daily supplementation over one to two weeks more consistently improves endurance markers and oxygen efficiency. If you’re serious about getting the most from citrulline, daily use rather than occasional pre-workout dosing is the stronger approach.
Side Effects and Tolerability
Citrulline is well tolerated at standard doses. Single doses of 3 to 6 grams rarely cause any side effects. The main concern at higher doses is gastrointestinal discomfort: nausea, bloating, or diarrhea. In healthy athletes, symptoms tend to appear at single doses above 9 grams. These issues occur because large doses of amino acids that aren’t fully absorbed in the small intestine can draw water into the gut. Staying at or below 8 grams per dose keeps most people comfortable. If you’re stacking citrulline with other amino acid supplements like arginine, keep the combined dose in mind, as the GI effects are additive.