L-Citrulline and Blood Flow: What the Research Actually Used and Found

Why citrulline shows up in men’s formulas

Every page, every journal post, the bottle facts and the ingredients go through review by Dr. Shawn Blick, MD before they ship. Walk through the men’s aisle of any supplement store and you will see the same word again and again: nitric oxide. Pre-workout powders promise “pump.” Vitality gummies promise “performance.” Very often, the ingredient behind both is L-citrulline.

The reason is simple. Nitric oxide is a gas the body makes to relax the smooth muscle in blood-vessel walls. Relaxed vessels widen and carry more blood. More blood flow to working muscles may help exercise. More blood flow is also central to how erections work.

You cannot swallow nitric oxide. But you can take the raw materials the body uses to make it. That is where citrulline comes in.

How citrulline becomes nitric oxide

Citrulline is an amino acid. Your body makes some on its own, and you get more from foods like watermelon. After you absorb it, your kidneys convert citrulline into another amino acid, arginine. Cells then use arginine to make nitric oxide (Cleveland Clinic, “L-Citrulline Supplement Benefits and Side Effects”).

You might ask: why not just take arginine? The short answer is that a lot of swallowed arginine is broken down in the gut and liver before it reaches the blood. Citrulline slips past that first pass more easily, then becomes arginine later. That is why many formulas now choose citrulline.

What the exercise studies used and found

The Cleveland Clinic summary we checked describes a study in which cyclists took 2.4 grams of citrulline a day for eight days. They rode faster and felt less tired afterward.

But the same summary also notes that a recent review found no clear answer on whether citrulline gives a real competitive edge. Benefits looked stronger for strength training than for endurance work. In other words, a few studies are encouraging, and the overall picture is still mixed.

What the erection study used and found

The best-known study on citrulline and erections was small. Men with mild erectile problems took 1.5 grams a day for a month. About half of them reported a benefit.

That is interesting, but it is one small study. The Cleveland Clinic article sums it up fairly: the jury is still out. Citrulline is not a replacement for prescription ED medicine, and it should not delay a medical checkup.

The dose problem in gummies

Look at the numbers above: 1.5 grams, 2.4 grams, 3 to 6 grams. Those are big amounts. A gram is 1,000 milligrams.

Many gummies weigh only a few grams each, and most of that weight is the gummy itself: sugar or sweetener, a gelling agent, flavor and color. When a gummy also contains several herbs, it may be hard to fit gram-level citrulline in a normal daily serving.

We are not saying any product is under-dosed. We are saying you should check. HorseFil, for example, names L-citrulline on its product image but does not publish the amount, so we cannot compare it with these studies.

Side effects and interactions

At common doses, citrulline is generally well tolerated. The Cleveland Clinic summary lists stomach-related effects such as bloating, cramping and diarrhea, plus sweating.

The bigger concern is combining it with medicines that also lower blood pressure or work on blood flow. Men who take nitrates for chest pain, ED drugs, or blood-pressure medicine should talk to a doctor before taking any nitric-oxide booster.

HorseFil names L-citrulline among six ingredients. See what is inside before you order.

Check today’s price

Food first: watermelon and friends

Watermelon is the classic food source of citrulline; the flesh and especially the rind contain it. You would need to eat a lot of watermelon to reach study doses, but it is a healthy, hydrating food anyway.

The broader point: foods and habits that help blood vessels also help energy and performance. Vegetables, fruit, fish, nuts, regular exercise and no smoking all support healthy vessels.

• Watermelon (citrulline)
• Leafy greens and beets (nitrates from food)
• Fish and nuts (healthy fats)
• Daily brisk walking

Who might consider citrulline, and who should not

Healthy adult men who train, or who want to support healthy blood flow as part of a wider routine, are the usual audience. For them, citrulline at common doses is generally well tolerated.

The picture changes for men who already take something that acts on blood vessels. Nitrates for chest pain, ED drugs and blood-pressure medicine all work on the same system. Adding a nitric-oxide booster on top without advice is not a good idea. Men with low blood pressure, or who often feel dizzy when they stand up, should also be careful.

When in doubt, a pharmacist can check your medicine list against a supplement label in a few minutes. That quick check is worth more than any online review.

Citrulline vs. arginine vs. nitrates

All three aim at nitric oxide. None should be combined with nitrate heart medicine or ED drugs without a doctor’s advice.

The bottom line

L-citrulline is a reasonable ingredient with a clear mechanism: it feeds the body’s nitric-oxide system. The research is small and mixed, and it used gram-level doses.

If you choose a product with citrulline, check the amount on the label, keep expectations modest, and talk to a doctor first if you take heart, blood-pressure or ED medicine.

Sources

1. Cleveland Clinic. L-Citrulline Supplement Benefits and Side Effects. Devon Peart, RD, February 7, 2023. Checked September 22, 2026.
2. NIH Office of Dietary Supplements. Carnitine: Fact Sheet for Health Professionals. NIH ODS, current revision. Checked September 22, 2026.