CitrullineAmino Acid

• Blood pressureLikely helps

  A meta-analysis of randomised trials found L-citrulline lowered systolic blood pressure by about 4 mmHg against placebo [2].
  
  Diastolic pressure moved the same direction but did not reach significance overall — only in the subgroup taking 6 g/day or more [2].
  
  A separate systematic review of 12 RCTs in 360 postmenopausal women, all US-based, found the same modest systolic reduction at 6–10 g/day over 4 to 8 weeks [34]. The effect was clearest in women who already had hypertension. ^[2]

• Exercise performancePossibly helps

  The largest pooling disagrees with the study everyone quotes.
  
  A 2026 systematic review and three-level meta-analysis of 30 RCTs (644 participants, 138 effect sizes) found only small, context-dependent improvements, with GRADE certainty running low to very low [13]. The 8 g subgroup reached significance at Hedges' g = 0.20 (95% CI 0.02–0.38); the 12 g and 15 g subgroups did not.
  
  The famous single study found a single 8 g dose of citrulline malate raised repetitions to fatigue by up to 52.9% in the final bench-press set [11].
  
  An independent sports-science commentary calls the measured effects 'very small', with several studies finding none at all [12]. Sex, training status, dose and timing did not moderate the pooled result [13]. ^[13]

• Muscle soreness after resistance trainingPossibly helps

  The original 8 g citrulline malate study reported soreness down 39.7% at 24 hours and 41.8% at 48 hours against placebo [11].
  
  A critical review of the citrulline malate literature found this partly replicated in later studies, but not universally [10]. One 6 g single-dose study in 9 men found no significant change in soreness. ^[10]

• Mild erectile dysfunctionPossibly helps

  A single-blind trial in 24 men with mild ED used 1.5 g/day for one month [6].
  
  Erection hardness score moved from 'mild ED' to 'normal' in 50% of participants, against 8.3% during the placebo month (p<.01), and monthly intercourse frequency rose [6]. The authors note it appeared less effective than PDE5 inhibitors such as sildenafil over the short term.
  
  A randomised crossover pilot found adding L-citrulline plus trans-resveratrol on top of existing PDE5-inhibitor therapy improved function in men unsatisfied with the drug alone [8].
  
  A three-arm trial comparing 1,500 mg/day citrulline, tadalafil 5 mg/day and the combination was still running as of this research, with no results yet [7]. No large randomised trial has established safety for this use specifically, and combining it with other vasodilatory drugs is discouraged [9]. ^[6]

• Muscle strength in postmenopausal womenPossibly helps

  In the 12-RCT review of 360 postmenopausal women, strength benefits showed up mainly when citrulline was paired with resistance training [34].
  
  No adverse effects were reported across the included studies. Citrulline alone did not carry the strength result. ^[34]

• PreeclampsiaPossibly helps

  Be careful with the species here. A mouse model of preeclampsia found citrulline during pregnancy improved maternal vascular function — mouse, not human [33].
  
  The one existing human L-citrulline RCT, in 36 women with chronic hypertension, showed no effect on preeclampsia risk or blood pressure [32]. That review found the arginine evidence base considerably larger than citrulline's.
  
  A larger multicentre randomised trial tested oral L-citrulline in 115 women with preeclampsia to see whether delivery could be delayed [31]. ^[32]

• L-citrulline (the free amino acid)Blood-pressure and general circulatory research, which used this form rather than the malate .

  Elemental content

  The amino acid on its own, with no counter-molecule attached

  Absorption

  Well absorbed orally. 3 g/day is enough to measurably raise plasma L-arginine [1].
  
  Dose-ranging work found it tolerated up to 15 g/day in healthy volunteers [1].

  Best for

  Blood-pressure and general circulatory research, which used this form rather than the malate [2].

• Citrulline malateNearly all the performance research used this form, so it is what that evidence actually covers .

  Elemental content

  L-citrulline bound to malic acid, sold at a nominal 2:1 citrulline-to-malate ratio

  Absorption

  No human trial has compared it against plain L-citrulline at a matched citrulline dose [10].
  
  Independent testing of some commercial products found the real ratio closer to 1.1:1, meaning less citrulline per gram than the label implies [10].
  
  Marketing claims that malate improves bioavailability are not supported by any controlled human comparison in the sources reviewed here [13].

  Best for

  Nearly all the performance research used this form, so it is what that evidence actually covers [13].

• Citrulline HClNothing established. One unreviewed preprint is the entire evidence base for it .

  Elemental content

  A hydrochloride salt marketed for better water solubility

  Absorption

  A 2025 preprint single-dose comparison found roughly 25% lower relative citrulline bioavailability than plain L-citrulline at matched grams [22].
  
  The same study found about 2-fold better plasma arginine bioavailability at 2 g HCl than at 6 g L-citrulline. That suggests form may change the citrulline-to-arginine conversion, not just citrulline absorption.
  
  It is a preprint that has not completed peer review [22].

  Best for

  Nothing established. One unreviewed preprint is the entire evidence base for it [22].

• Gastrointestinal symptoms at high single doses

  Single doses of 3–6 g rarely provoked symptoms [4].
  
  Above roughly 9 g in one sitting, symptoms became more likely, and specifically in healthy athletes rather than in the diabetic patients in the same review [4]. The authors suggest that may reflect differences in gut motility.
  
  When effects do occur they are mild: bloating, cramping, diarrhoea, nausea, sweating [23]. ^[4]

• How it compares with arginine and ornithine

  A peer-reviewed safety review found no reported side effects from oral citrulline at doses up to 15 g, and identified no toxicity [21].
  
  Arginine and ornithine are the contrast — both can cause gastrointestinal side effects at around 10 g in a single bolus [21].
  
  Citrulline has also been used as long-term replacement therapy in children with urea cycle disorders [21]. ^[21]

• Pregnancy and breastfeeding

  Two clinical trials in pregnant women reported no serious adverse events at L-citrulline doses up to 3 g twice daily for 8 weeks [31].
  
  One congenital abnormality occurred in the placebo group, not the citrulline group, and was assessed as unrelated [31].
  
  For breastfeeding, no safety or efficacy data appear in the sources reviewed here at all. That gap is the reason for caution, not a reassurance [5]. ^[31]

• Nitrates and PDE5 inhibitors

  Citrulline becomes arginine, which is a nitric-oxide precursor, so additive vasodilation and hypotension are the mechanistic worry [24].
  
  The named drugs: nitroglycerin, isosorbide dinitrate/mononitrate, and PDE5 inhibitors sildenafil, tadalafil, vardenafil and avanafil [15].
  
  No dedicated controlled human interaction trial has tested any of this. Every source describes it as theoretical and not fully characterised, and severity is not established [9]. ^[9]

• Blood-pressure medication

  Same mechanism, same caveat: citrulline's own systolic effect of about 4 mmHg could add to an antihypertensive drug's [2].
  
  This has not been tested in a dedicated human interaction study either [24]. ^[24]

Watermelon is the richest known food source by a wide margin, and citrulline is named after the genus [25].

Content varies by cultivar and by tissue. One GC-MS analysis found a range of 3.9–28.5 mg/g dry weight, with rind at 24.7 mg/g against flesh at 16.7 mg/g [25].

On a fresh-weight basis flesh edges ahead — 1.9 against 1.3 mg/g — because rind holds less water [25].

Across the wider cucurbit family, watermelon dominates. 'Crimson Sweet' measured 2.85 g/kg fresh weight and 'Dixielee' 2.36–2.43 g/kg, against 0.86 g/kg for casaba-type melon and 0.45–0.64 g/kg for mouse melon or horned melon rind [28].

On a dry-weight basis: dessert watermelon about 34.2 g/kg, pumpkin 4.6, cantaloupe 3.78, cucumber 3.31, buffalo gourd 2.47 [28]. Citrulline was essentially undetectable in straightneck squash.

Per practical serving, a commercial estimate puts 1 cup of watermelon at about 365 mg [29]. A controlled comparison of 100-kcal servings gave rind 19.3 mg, flesh 10.0 mg and seeds 1.4 mg [26].

None of those servings changed flow-mediated dilation over 7 hours in that study [26]. Clinical trials use 3 to 15 g/day — thousands of milligrams — so matching them from watermelon alone is not realistic [1].

The food matrix may still help. In a Caco-2 human intestinal-cell model, watermelon skin showed greater citrulline transport than flesh, rind, or an L-citrulline standard, and unpasteurised watermelon juice outperformed the pure standard [27].

For exercise, the protocol is remarkably uniform: a single acute dose about 1 hour before training [14].

A 2020 systematic review found the most frequent regimen was 3–4 g of L-citrulline or 8 g of citrulline malate, taken roughly an hour pre-exercise [14].

The 2026 meta-analysis of 30 RCTs found the same pattern — about 8 g at around 60 minutes — and found ingestion timing was not a significant moderator of outcomes [13].

A critical review of the citrulline malate literature tabulates the same single 8 g pre-workout dose, occasionally with a post-exercise or once-daily maintenance dose in longer protocols [10].

For non-exercise use, no government or academic source reviewed here specifies a time of day. Splitting the dose across meals is commercial guidance, not a trial finding [5].