L-citrulline / citrulline malate

What it is

Citrulline is an amino acid made in the body and found in watermelon, where it gets its name. It sits in the urea cycle and is converted to arginine, which the body uses to make nitric oxide.

Supplements come as L-citrulline (pure) or citrulline malate (citrulline bonded to malic acid). It is widely used in pre-workouts, often at doses too low to matter.

People take it for blood flow, training endurance and reduced muscle soreness.

How it works

Taken orally, citrulline raises blood arginine more effectively than arginine itself does, because it bypasses much of the gut and liver breakdown. More arginine can support nitric oxide production, which relaxes blood vessels. Malate may play a role in energy metabolism, though this is less clear.

What the evidence says

• Limited Repetitions to failure in resistance training
  Some trials show a few extra reps, others show nothing; results vary by dose and study.
• Limited Reduced muscle soreness
  Possible small effect, not consistent.
• Limited Endurance exercise performance
  Small and inconsistent effects in studies.
• Moderate Blood pressure and vascular function
  Some trials show small reductions in people with elevated pressure; this is not a substitute for treatment.
• Weak Erectile function
  Small early studies only; not a replacement for medical assessment.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Citrulline malate: roughly 6 to 8 g taken 30 to 60 minutes before exercise is common in studies.
• Pure L-citrulline: 3 to 6 g is a typical range, with some studies using more. Check current guidance.
• Citrulline malate is often 2:1 citrulline to malate, so 8 g gives about 5.3 g citrulline.
• It is usually well tolerated on an empty stomach, but food can reduce stomach upset.

Forms and what to look for

• L-citrulline: pure, more citrulline per gram, usually flavour-neutral and slightly sour.
• Citrulline malate (2:1 is typical): the most common sports form; check the ratio on the label.
• Watermelon extract has far less citrulline per serving than most expect.
• Look for the stated grams of citrulline, not just blend totals.

What to expect, and when

• Blood levels rise within an hour of a dose.
• Any benefit to training is likely small; blood pressure effects, if any, would take weeks of daily use.

Side effects

• Generally well tolerated.
• Stomach discomfort, nausea or loose stools at higher doses.
• Occasional headache or flushing.
• Lower blood pressure may cause light-headedness in some.

Interactions

• Blood pressure medicines: additive lowering is possible, so check with a pharmacist.
• Nitrates, nitrate-containing heart medicines and erectile dysfunction drugs (such as sildenafil): possible additive blood vessel effects; ask a clinician.
• Other nitric oxide boosters such as arginine or beetroot may stack the effect.

Who should avoid it

• Anyone on blood pressure, angina or erectile dysfunction medication should check first.
• People with kidney or liver disease: speak with a clinician.
• Pregnant or breastfeeding people: limited data.
• Anyone with low blood pressure or fainting.

Risks and safety

• Not currently banned by WADA, but contamination risk exists in multi-ingredient products.
• Underdosed pre-workouts are common, so you may pay for little effect.
• Long-term safety data at high doses is limited.

Buying well: quality and testing

• Check grams of citrulline and the ratio (2:1, 1:1) on the label.
• Avoid proprietary blends where the amount is hidden.
• Third-party tested products (Informed Sport, NSF) are preferable for athletes.
• Single-ingredient powder is usually cheapest and clearest.

Bottom line

Citrulline is a reasonable, low-risk option for people who like a pre-workout and use a proper dose. Do not expect a dramatic change. If you take blood pressure or erectile dysfunction medication, speak to a pharmacist first.