Citrulline

What it is

Citrulline is an amino acid your body makes and turns into arginine, which in turn helps produce nitric oxide, a molecule that relaxes blood vessels [1]. It is sold as L-citrulline or as citrulline malate (citrulline bound to malic acid), and it is abundant in watermelon [2].

How it might work

The proposed route is more nitric oxide, which relaxes and widens blood vessels [1][3]. Citrulline is also thought to help buffer ammonia during hard exercise [4]. Unlike arginine taken by mouth, citrulline escapes most breakdown in the gut and liver and is converted to arginine afterwards [1]. These are proposed mechanisms, and the trials below do not prove them.

What the research says

Strength training. A meta-analysis (a study that pools many trials) of 8 trials with 137 people found that 6-8 g of citrulline malate taken 40-60 minutes before lifting added about 3 repetitions to failure, roughly 6% more than placebo [5]. A separate meta-analysis of 12 samples (198 people) found a small benefit in strength and power tests, with a pooled effect of 0.20 [3]. A 2026 meta-analysis of 30 trials (644 people) also found a small improvement in overall exercise performance (effect size 0.16), but rated the certainty of the evidence as low to very low [6]. One smaller meta-analysis of 4 trials in resistance-trained adults found no improvement in maximal muscle strength [7].

Endurance. Two meta-analyses did not find an endurance benefit: 10 studies of aerobic performance [8] and 9 studies (158 people) of time to exhaustion or time to finish [4].

Effort and soreness. A meta-analysis of 13 trials (206 people) found lower perceived exertion after exercise and less muscle soreness at 24 hours, but not at 72 hours, with no change in blood lactate [9]. A later meta-analysis of aerobic studies did not find a drop in perceived exertion [8].

Blood pressure. Results differ by review. A 2025 meta-analysis of 15 trials (415 people) in middle-aged and older adults, which pooled citrulline supplements and watermelon, found systolic pressure fell 4.02 mmHg and diastolic 2.54 mmHg [10]. An earlier meta-analysis of five interventions found no significant change in brachial blood pressure [11]. A review of 12 trials in 360 postmenopausal women reported that most blood pressure studies showed reductions, up to 9 mmHg in systolic pressure in some studies [12].

Erectile function. In a single-blind study of 24 men with mild erectile dysfunction, 1.5 g a day for a month moved 12 men (50%) to normal erection hardness, compared with 2 men (8.3%) during the earlier placebo month [1]. This is the only citrulline-alone human study we found, and it was small and short.

Blood sugar. In a placebo-controlled trial of 45 people with overweight or obesity and type 2 diabetes, 3 g a day for 8 weeks lowered fasting glucose, insulin, HbA1c (a three-month blood sugar marker) and HOMA-IR (an insulin resistance score) [13].

Body composition. A meta-analysis of 21 trials found no overall effect on weight, BMI, fat mass, waist size or fat-free mass. Subgroup findings, such as less fat mass in people over 40 and in those taking more than 6 g a day, were exploratory [2].

Doses used in studies

For exercise, trials most often used 8 g of citrulline malate taken shortly before exercise [9]. The strength-training meta-analysis covered 6-8 g of citrulline malate taken 40-60 minutes before training [5]. The erectile function study used 1.5 g a day of L-citrulline for one month [1]. The type 2 diabetes trial used 3 g a day for 8 weeks [13]. The postmenopausal women reviewed in one systematic review were studied for 4-8 weeks [12]. These are the doses studied, not recommendations.

Safety and interactions

Side effects. None of the 12 trials in postmenopausal women reported adverse effects related to citrulline [12]. The 24 men in the erectile function study finished without adverse events [1].

Upper limits. A review of human dose studies in relatively healthy adults gave a no-observed-adverse-effect level for citrulline of 24 g a day [14]. No official upper intake level has been set.

Medicines. We found no interaction studies. Trials in older adults suggest citrulline may lower blood pressure, but its use alongside blood pressure medicines was not studied in the reviews we read [10]. In a small crossover study, men took citrulline with resveratrol alongside on-demand erectile dysfunction drugs (PDE5 inhibitors) and the 13 men who finished reported no adverse events [15].

Pregnancy and breastfeeding. A meta-analysis of arginine and citrulline in pregnancy found only one citrulline study (36 women), which reported no effect on pre-eclampsia or blood pressure; no safety conclusion can be drawn from it [16]. We found no data for breastfeeding.

Kidney, liver and other conditions. We found no data on people with kidney or liver disease. The safety data above come from healthy adults or people in the specific trials [14].

Caveats and open questions

Most exercise trials are small, and the 2026 meta-analysis rated the certainty of its findings as low to very low, with wide prediction intervals and no firm answer on best dose or timing [6]. Reviews disagree on perceived exertion and on blood pressure [8][11]. The blood pressure meta-analysis that found a drop mixed citrulline supplements with watermelon, so it does not isolate the supplement [10]. A trial of L-citrulline plus tadalafil for mild to moderate erectile dysfunction (133 men, NCT07642505) is recruiting and may add better data.