L-ArginineAmino Acid

• Blood pressureLikely helps

  A 2022 dose-response meta-analysis of 22 RCTs found oral L-arginine lowered systolic pressure by 6.40 mmHg and diastolic by about 2.64 mmHg versus placebo [9].
  
  The systolic effect became significant around 4 g/day and tailed off above 9 g/day.
  
  An earlier 2011 meta-analysis of 11 RCTs at 4–24 g/day landed close: systolic −5.39 mmHg, diastolic −2.66 mmHg [10]. Mayo Clinic notes the effect appears stronger in women than in men [7]. ^[9]

• Erectile dysfunctionPossibly helps

  Mayo Clinic grades the evidence for oral L-arginine in ED as a C — unclear whether it helps [7].
  
  Studies find it generally safe and well tolerated, but unlikely on its own to restore erectile function. Combinations with yohimbine or pycnogenol look more promising [8].
  
  Some consumer health content is more upbeat than Mayo is. We're taking the more cautious clinical-reference read as primary. ^[7]

• Athletic performance — two reviews, opposite verdictsPossibly helps

  NIH ODS concludes that 2–20 g/day has shown little to no effect on aerobic or anaerobic performance in most trials, and no consistent effect on blood flow or exercise metabolites [1].
  
  A 2020 systematic review and meta-analysis of 18 studies found the opposite — significant improvement in both aerobic and anaerobic markers [11].
  
  Its effective protocols were about 0.15 g/kg taken 60–90 minutes pre-exercise, or 1.5–2 g/day chronically for aerobic gains and 10–12 g/day for 8 weeks for anaerobic gains [11].
  
  Both stay on this page. They differ in scope and study-inclusion criteria rather than flatly contradicting each other. ^[1]

• Growth hormonePossibly helps

  Most studies of oral arginine on its own show a real rise in resting growth hormone, some reports putting it over 100% above baseline [30].
  
  Add exercise and the picture inverts. The arginine-plus-exercise GH response tends to be blunted compared with exercise alone, which is itself a potent GH stimulus [1].
  
  Whether any GH rise turns into more muscle blood flow or actual protein synthesis is unresolved [1]. ^[30]

• Pre-eclampsia risk in at-risk pregnanciesPossibly helps

  A 2025 systematic review in BJOG found low-certainty evidence that L-arginine may lower the risk of pre-eclampsia and severe pre-eclampsia in at-risk pregnancies [34].
  
  Low-certainty is the review's own grading, not our hedge. This was supervised trial use, not an over-the-counter decision. ^[34]

• Peripheral arterial disease and anginaPossibly helps

  Mayo Clinic lists both among uses with preliminary or mixed evidence [7].
  
  Neither is firmly established, and both are described as needing physician oversight rather than self-directed use [8]. ^[7]

• Nothing applies to healthy adults on a normal protein-containing diet — no distinct low-arginine syndrome is described there [5]
• Arginine turns conditionally essential during physiological stress: critical illness, sepsis, burns, trauma, major surgery, infection, cancer, and rapid growth in infants and children [13]
• Slower wound repair, reported in the critical-care and surgical literature during those stress states [14]
• Reduced T-cell and immune function, alongside worse postoperative outcomes — the reason arginine-enriched formulas appear in some surgical nutrition [14]
• In sepsis, disrupted arginine metabolism with impaired microcirculation and haemodynamic instability. Some researchers describe sepsis itself as an "arginine deficiency state" [12]
• Groups flagged for inadequate status: protein malnutrition, burns, active infection, and rapid growth needs [15]^[13]

• L-arginine free baseThe form most of the controlled research used, including the blood-pressure trials .

  Elemental content

  Plain L-arginine, no salt or conjugate. It is hygroscopic and can degrade in humid conditions [17].

  Absorption

  Poor, and this is the central practical fact about arginine.
  
  A pharmacokinetic study put absolute oral bioavailability of a single 10 g dose at about 20%, from extensive first-pass metabolism in the gut and liver [16].

  Best for

  The form most of the controlled research used, including the blood-pressure trials [17].

• L-arginine hydrochloride (HCl)The most common commercial form.

  Elemental content

  A salt form. The hydrochloride portion is roughly 17% of the molecular weight, so "1,000 mg L-arginine HCl" delivers less free arginine than the label number implies [17].

  Absorption

  More water-soluble and more stable than the free base [17].
  
  The bioavailability limit is unchanged, so compare products on free-arginine content rather than salt weight.

  Best for

  The most common commercial form. Do the 17% arithmetic before comparing two labels [17].

• Arginine alpha-ketoglutarate (AAKG)Nothing the plain form doesn't do, on current evidence.

  Elemental content

  Arginine paired with alpha-ketoglutarate, a Krebs-cycle intermediate [17].

  Absorption

  Marketed heavily in pre-workout products for nitric-oxide "muscle pumps" [18].
  
  Clinical evidence that AAKG does anything beyond plain L-arginine is lacking [17].

  Best for

  Nothing the plain form doesn't do, on current evidence. The controlled research behind arginine's effects used plain L-arginine [17].

• L-citrulline (the standard alternative)Raising plasma arginine, if that is the actual goal.

  Elemental content

  A different amino acid, converted into arginine mainly in the kidneys [19].

  Absorption

  Absorbed more efficiently, and it bypasses most of the first-pass liver breakdown that caps arginine [19].
  
  In some comparisons it raises circulating arginine more effectively than an equivalent dose of arginine itself.

  Best for

  Raising plasma arginine, if that is the actual goal. It is not the form the arginine trials tested [33].

• Gut symptoms above roughly 9 g/day

  Bloating, diarrhoea, nausea and stomach discomfort are the usual complaints [1].
  
  They grow more likely as the dose climbs past about 9 g/day [3]. ^[1]

• After a heart attack — a contraindication, not a caution

  Health Canada's natural health product monograph contraindicates arginine products supplying more than 9 g/day in anyone who has had a myocardial infarction [20].
  
  That rests on the randomised VINTAGE MI trial, which gave 9 g/day of oral L-arginine after acute ST-elevation MI [21].
  
  It was stopped early. Mortality reached 8.6% in the arginine group against zero deaths in the control group [21]. ^[20]

• Kidney impairment and raised potassium

  Arginine has been linked to electrolyte disturbances including hyperkalaemia, particularly by IV infusion and at high doses [20].
  
  Caution applies in renal impairment, diabetes and hepatic impairment, and alongside potassium-sparing diuretics [22].
  
  A review of renal safety in athletes found both beneficial and detrimental effects on kidney function, with adverse effects uncommon across 3 g/day to over 100 g/day [32]. Existing kidney disease still means medical supervision rather than self-dosing. ^[20]

• Cold sores and genital herpes

  Herpes simplex needs arginine as a substrate to replicate, and lysine competitively blocks that in laboratory and cell studies [23].
  
  On that mechanism, Health Canada's monograph and some clinicians advise caution with high arginine intake in people prone to outbreaks [20].
  
  The caution is extrapolated from mechanistic and dietary-pattern work. No large randomised trial has tested whether arginine supplements actually set outbreaks off in people [23]. ^[20]

• Pregnancy

  Not a self-supplementation decision. Health sources consistently ask for physician guidance rather than over-the-counter use during pregnancy [34].
  
  The pre-eclampsia evidence that exists is low-certainty and came from supervised trials in at-risk pregnancies [34]. ^[34]

• Nitrates and other nitric oxide donors

  Nitroglycerin, isosorbide mononitrate and dinitrate, sodium nitroprusside [22].
  
  Arginine drives vasodilation through nitric oxide itself, so stacking it on a nitrate can lower blood pressure additively into hypotension [19]. Professional drug-interaction references flag this one specifically [21]. ^[22]

• PDE5 inhibitors — sildenafil, tadalafil

  These work on the same nitric oxide pathway arginine feeds [19].
  
  The combination risks additive blood-pressure lowering. That is exactly the scenario someone self-experimenting with arginine for ED walks into [21]. ^[19]

• Blood pressure medicines

  ACE inhibitors, ARBs, calcium channel blockers and diuretics [19].
  
  Arginine's own blood-pressure effect is measurable and meta-analysed, so assume the effects can stack. ^[19]

• Insulin and glucose-lowering drugs

  Clinical references describe arginine's effect on insulin and diabetes medication as unpredictable [19].
  
  That is the word they use. Blood sugar warrants monitoring where the two are combined [21]. ^[19]

Arginine shows up wherever there's protein, but it concentrates in seeds, nuts, legumes and soy. Meat, seafood and dairy come next [6].

Pumpkin seeds are the standout, at roughly 5.1–5.4 g per 100 g depending which table you read [24].

Other plant sources per 100 g: peanuts 3.3–3.5 g, walnuts 3.6 g, spirulina 4.2 g, soy products 2.4–4.6 g, almonds 2.5–2.8 g, lentils 2.2 g [25].

The animal-food figures disagree between sources, and that's worth naming rather than averaging away. One table lists beef at 1.54 g/100g, chicken breast 1.35 g, shrimp 1.74 g [25].

Another puts beef and shrimp near 2.25–2.5 g/100g and chicken at 2.17 g [24]. Different databases, cuts or cooking methods, most likely — neither is a government nutrient database.

Across plant foods, arginine runs about 3% to 15% of total protein [6]. Soy, peanuts, walnuts and fish sit at the rich end; cereal proteins at the poor end, around 3–4%.

Watermelon isn't much of an arginine source itself. It is the best-known dietary source of L-citrulline, which the body converts into arginine — the point USDA's Agricultural Research Service built its own coverage around [26].

No dedicated trial has compared times of day for L-arginine. Most timing advice traces back to supplement-industry and consumer health sources rather than controlled research [19].

The one consistent recommendation is an empty stomach, or between meals [29]. Single amino acids are thought to compete with dietary protein for the same intestinal transporters, so a mixed meal gets in the way [19].

For exercise there is genuine trial-backed timing. The 2020 meta-analysis found an acute window of 60–90 minutes before exercise, at about 0.15 g/kg, was the protocol linked to measurable performance gains [11].

For blood pressure and everything else, nothing separates morning from evening. That's an evidence gap, not a finding.