L-citrulline and L-arginine are amino acids involved in nitric oxide production. Nitric oxide helps widen blood vessels, improves blood flow, and plays a key role in erections, endurance, and recovery. You can get both from food, but they’re often taken as supplements—especially by men looking to improve sexual performance or boost workout energy.

Let’s look at what research says, how each one works, and which might actually be worth taking.

Why It Matters for Men

Erections and Blood Flow
Erections depend heavily on blood flow. Nitric oxide relaxes the smooth muscle in blood vessels, allowing more blood to enter the penis. That’s the same mechanism used by prescription meds like Viagra. Citrulline and arginine work through that same pathway—just more gently.

In one small study, 24 men with mild erectile dysfunction took 1.5g of L-citrulline daily. After one month, half of them returned to normal erectile function. The effect wasn’t instant, but it was consistent.
Study on PubMed

L-arginine has also been studied—particularly in men with vascular-related ED. A six-month study using 6g per day showed improved function, especially in moderate cases.
Study on Springer

Endurance and Exercise
Nitric oxide improves oxygen and nutrient delivery to muscles. That means better stamina and faster recovery.

L-citrulline at 3–6g per day has been shown to improve VO₂ kinetics and reduce fatigue in athletes.
Review on PMC

L-arginine might help too, but it doesn’t absorb as reliably.

L-Citrulline vs L-Arginine — What’s the Difference?

• How they work:
  Arginine is directly used in nitric oxide synthesis. Citrulline first converts into arginine, then helps produce nitric oxide.
• Absorption:
  Arginine is partly broken down in the digestive system. Citrulline is absorbed more reliably and leads to higher blood arginine levels.
• Onset of effect:
  Arginine acts faster, but less predictably. Citrulline takes longer, but is more stable.
• Side effects:
  Arginine is more likely to cause bloating or stomach discomfort. Citrulline is generally easier to tolerate.
• For erectile function:
  Both can help with mild to moderate ED. But citrulline tends to be more consistent across users.
• For endurance and recovery:
  Citrulline is commonly used in pre-workout supplements and seems more reliable for boosting stamina.

Which One Should You Choose?

If you’re focused on endurance, recovery, or general health, L-citrulline is a safer bet. It absorbs better, works more predictably, and causes fewer gut issues.

If you’re looking for a fast-acting boost—say, before sex—L-arginine might give you a quicker response. But its effects vary more from person to person.

Some supplements combine both, which can offer short-term and sustained effects.

Do You Even Need a Supplement?

Both amino acids occur naturally in food—especially meat, fish, legumes, nuts, seeds, and watermelon. If your diet is solid, your baseline levels might be enough. But if you’re under stress, sleeping poorly, exercising hard, or dealing with mild ED, supplements could help fill the gap.

A Few Quick Notes on Use

• Start with a low dose—around 2–3g per day—and see how your body reacts.
• Don’t expect instant results. These aren’t pharmaceutical-grade fixes.
• If you’re on blood pressure meds, talk to a doctor before adding these. They can lower blood pressure.

Final Take

Both amino acids affect the same system but behave differently. Arginine is faster but unpredictable. Citrulline is slower but steadier. If you’re choosing just one for long-term support in men’s health, L-citrulline is usually the better option—especially if you want consistency over short-term results.