L-citrulline is a naturally occurring amino acid that plays an important role in the body's production of L-arginine and nitric oxide. Nitric oxide is involved in blood vessel relaxation, circulation, and several processes related to exercise and cardiovascular function.

L-citrulline is naturally produced by the body and can also be obtained from foods such as watermelon. Today, L-citrulline is widely available as a dietary supplement and is commonly used in sports nutrition and wellness products.

But what does L-citrulline actually do? Does L-citrulline improve blood flow? How much L-citrulline should you take? Can L-citrulline support exercise performance or erectile function? And is L-citrulline safe?

Research has investigated L-citrulline across several areas, including nitric oxide production, blood pressure, exercise performance, vascular function, and erectile function. Some findings are promising, while other areas remain mixed or inconclusive.

Key Takeaways

• L-citrulline is a nonessential amino acid naturally produced by the body and found in foods such as watermelon.
• L-citrulline can be converted into L-arginine, which serves as a substrate for nitric oxide production.
• Nitric oxide plays an important role in blood vessel relaxation and vascular function.
• Research has investigated L-citrulline for blood flow, blood pressure, exercise performance and erectile function.
• Evidence for blood-pressure effects is mixed, with some meta-analyses reporting reductions and others finding no significant overall effect.
• Research on exercise performance has produced mixed results. Some studies report improvements in selected performance measures, while broader analyses have found limited effects on aerobic performance.
• A small clinical trial found improved erection hardness among men with mild erectile dysfunction following 1.5 g of L-citrulline daily.
• L-citrulline and citrulline malate are the two common forms found in dietary supplements.
• Research has commonly investigated approximately 3–6 g of L-citrulline per day, while citrulline malate is often provided at higher total doses because the product also contains malate.
• Short-term studies suggest that L-citrulline is generally well tolerated, although long-term safety data remain more limited.
• People taking medication for blood pressure, cardiovascular conditions, or other health concerns should speak with a healthcare professional before using L-citrulline supplements.

What Is L-Citrulline?

L-citrulline is a nonessential amino acid, meaning the human body can produce L-citrulline naturally. Unlike amino acids that are directly incorporated into proteins, L-citrulline participates in metabolic pathways including the urea cycle.

One of the most important reasons L-citrulline is studied as a supplement is the relationship between L-citrulline, L-arginine, and nitric oxide.

After L-citrulline is absorbed, the body can convert L-citrulline into L-arginine. L-arginine can then serve as a substrate for nitric oxide synthase, an enzyme involved in nitric oxide production. Nitric oxide helps regulate vascular tone and can promote relaxation of blood vessel smooth muscle.

L-citrulline is naturally found in foods including:

• Watermelon
• Pumpkin
• Cucumber
• Gourds
• Bitter melon

Watermelon is one of the better-known dietary sources of L-citrulline.

L-citrulline supplements are generally available in two forms:

• L-citrulline: Citrulline provided without another compound.
• Citrulline malate: Citrulline combined with malate.

These forms should not automatically be considered equivalent because the amount of actual L-citrulline differs between formulations.

How Does L-Citrulline Work?

The L-citrulline pathway is closely connected to nitric oxide production.

A simplified pathway looks like this:

L-Citrulline → L-Arginine → Nitric Oxide → Vascular Relaxation

L-citrulline can increase circulating L-arginine levels, which may support nitric oxide production.

One reason researchers have investigated L-citrulline instead of L-arginine directly is the way the body absorbs and metabolizes the two amino acids.

Oral L-arginine undergoes substantial metabolism before reaching systemic circulation, while L-citrulline can be converted into L-arginine through metabolic pathways. This difference may help explain why L-citrulline can increase circulating L-arginine levels despite not being directly converted into nitric oxide.

Individual response depends on factors including dose, health status, baseline nutrient levels, and the outcome being measured.

What Are the Potential Benefits of L-Citrulline?

1. L-Citrulline and Nitric Oxide

The relationship between L-citrulline and nitric oxide is one of the main reasons L-citrulline is used in dietary supplements.

L-citrulline can increase circulating L-arginine, which provides substrate for nitric oxide production.

Nitric oxide plays a role in:

• Blood vessel relaxation
• Vascular tone
• Blood flow
• Endothelial function
• Exercise-related circulation

A clinical study involving 15 middle-aged men found that seven days of 5.6 g of L-citrulline per day increased nitric-oxide-related metabolites and circulating L-arginine while also reducing a measure of arterial stiffness. Original clinical trial

However, the study involved only 15 participants, so the findings should be interpreted cautiously.

Bottom line: L-citrulline participates in the L-arginine-nitric oxide pathway, but changes in nitric oxide-related markers do not automatically establish a specific health outcome.

2. L-Citrulline and Blood Pressure

Blood pressure is another area that has received considerable research attention. The evidence is mixed.

A systematic review and meta-analysis of eight clinical trials found that L-citrulline supplementation was associated with a modest reduction in systolic blood pressure. The included studies used approximately 3–9 g of L-citrulline per day and lasted between one and 17 weeks. Original systematic review and meta-analysis

Another systematic review and meta-analysis found no significant overall effect on blood pressure. Original systematic review and meta-analysis

Differences in participant characteristics, baseline blood pressure, dosage, study duration, and formulation may help explain the inconsistent findings.

Bottom line: L-citrulline may influence blood pressure in some populations, but current evidence is not consistent enough to treat L-citrulline as a substitute for blood-pressure medication or medical care.

L-Citrulline and Blood Flow

L-citrulline receives attention for blood flow because of the connection between L-citrulline, L-arginine, and nitric oxide.Nitric oxide contributes to vascular relaxation, which can influence circulation. Some clinical research has reported improvements in vascular measures following L-citrulline supplementation.

For example, a randomized clinical trial involving 25 older adults found that 14 days of 6 g of L-citrulline per day increased exercise blood flow and vascular conductance in older men, although the same changes were not observed in older women. Original clinical trial

However, not every study has produced similar findings.

A clinical trial involving older men found that a high dose of citrulline did not increase muscle blood flow or microvascular circulation following resistance exercise. Original clinical trial

Bottom line: L-citrulline can influence pathways involved in vascular function, but research does not establish a uniform effect on blood flow across all populations and situations.

L-Citrulline and Exercise Performance

L-citrulline is commonly included in sports and pre-workout supplements because of its relationship with nitric oxide and blood flow.

Researchers have investigated L-citrulline and citrulline malate for both endurance and resistance exercise.

Endurance Exercise

A systematic review and meta-analysis of 10 studies examined citrulline supplementation and aerobic exercise outcomes.

The analysis found no significant overall improvement in aerobic performance, perceived exertion, VO₂ kinetics, or lactate. Original systematic review and meta-analysis

These findings suggest that the theoretical connection between L-citrulline and exercise physiology does not necessarily translate into a measurable improvement in every endurance setting.

Resistance Training

Some individual studies involving citrulline malate have reported improvements in resistance-training performance.

For example, one randomized trial involving 41 men found that participants completed more repetitions during upper-body resistance exercise after consuming 8 g of citrulline malate compared with placebo. Original clinical trial. Other studies have reported reductions in muscle soreness or improvements in selected measures of resistance-training performance.

However, results across studies are not uniform.

The distinction between L-citrulline and citrulline malate also matters because different studies use different formulations and doses.

Bottom line: L-citrulline and citrulline malate have been studied extensively in sports nutrition, but the overall evidence remains mixed. Individual studies have reported performance benefits, while broader analyses have found limited or inconsistent effects.

Does L-Citrulline Increase Testosterone?

L-citrulline is sometimes included in male-performance supplements, but the primary research interest surrounding L-citrulline involves nitric oxide, blood flow, and vascular function rather than testosterone.

A small dose-ranging study involving healthy men examined doses of 2, 5, 10, and 15 g of L-citrulline.

The researchers measured several hormones and found no significant effect on growth hormone or insulin following acute L-citrulline administration. Original dose-ranging study

Current evidence does not establish L-citrulline as an ingredient that reliably increases testosterone levels.

Bottom line: L-citrulline research primarily concerns nitric oxide, vascular function, and exercise-related outcomes rather than testosterone regulation.

L-Citrulline vs. L-Arginine: What's the Difference?

L-citrulline and L-arginine are closely connected but are not the same amino acid.

L-arginine can be directly used by nitric oxide synthase to produce nitric oxide. L-citrulline can first be converted into L-arginine, increasing the body's circulating L-arginine pool.

The difference becomes particularly relevant when comparing supplements.

Some research suggests that oral L-citrulline can raise circulating L-arginine more effectively than equivalent oral L-arginine because of differences in absorption and metabolism.

This does not mean that L-citrulline is universally better than L-arginine. The appropriate choice depends on the formulation, dose, intended use, and individual circumstances.

Bottom line: L-citrulline and L-arginine both participate in the nitric oxide pathway, but the body processes the two amino acids differently.

L-Citrulline vs. Citrulline Malate: What's the Difference?

The two most common forms of citrulline supplements are L-citrulline and citrulline malate.

L-citrulline contains citrulline as the primary active amino acid. Citrulline malate combines citrulline with malate, a compound involved in cellular energy metabolism.

The distinction matters because the total amount listed on a citrulline malate supplement does not represent pure L-citrulline.

For example, approximately 1.75 g of citrulline malate may provide around 1 g of L-citrulline, depending on the formulation and citrulline-to-malate ratio.

When comparing research studies or supplements, always check whether the study used L-citrulline or citrulline malate and determine how much actual citrulline the serving provides.

Bottom line: L-citrulline and citrulline malate are related formulations, but the two should not be compared based only on total serving weight.

How Much L-Citrulline Should You Take?

There is no single universally established optimal dose of L-citrulline.

Research has used different doses depending on the intended outcome.

Commonly studied amounts include:

• 3–6 g per day of L-citrulline
• Approximately 6–8 g of citrulline malate in some exercise studies
• Higher doses have also been investigated in safety and pharmacokinetic research

A systematic review of blood-pressure studies included L-citrulline doses ranging from 3–9 g per day. Original systematic review and meta-analysis

Research involving mild erectile dysfunction used 1.5 g per day. Original clinical trial

Exercise studies have used different amounts depending on whether researchers were evaluating endurance, resistance exercise, oxygen utilization, or other outcomes. Therefore, a dose used in one clinical trial should not automatically be considered an ideal daily dose for every person.

The appropriate amount depends on:

• The form of citrulline
• The amount of actual L-citrulline per serving
• The intended use
• Individual health circumstances
• Other supplements or medications
• Duration of supplementation

When Should You Take L-Citrulline?

Timing depends on the purpose of supplementation.

Exercise studies have commonly examined L-citrulline or citrulline malate before exercise, with some protocols using the supplement approximately 30–60 minutes before physical activity.

Other research has used daily supplementation for several days or weeks.

For example, the clinical trial involving mild erectile dysfunction used 1.5 g of L-citrulline daily for one month. Original clinical trial

Research involving arterial stiffness used 5.6 g per day for seven days. Original clinical trial

These differences show why there is no single timing strategy that applies to every L-citrulline supplement.

Bottom line: Pre-exercise use is common in sports nutrition research, while other health outcomes have been studied using daily supplementation over longer periods.

What Are the Side Effects of L-Citrulline?

L-citrulline appears to be generally well tolerated in short-term research.

A small pharmacokinetic study examined single doses of 2, 5, 10, and 15 g of L-citrulline in healthy men and reported no side effects. Original dose-ranging study

A separate four-week tolerance study examined daily citrulline doses of 6, 12, 18, and 24 g in healthy adult men. The researchers reported that participants tolerated the supplementation protocols without treatment-related adverse events.

However, individual tolerance can vary.

Potential concerns may include:

• Gastrointestinal discomfort
• Changes in blood pressure
• Interactions with medications that affect blood pressure or vascular function

Long-term safety data remain more limited than short-term safety data.

Who Should Be Careful With L-Citrulline?

People with certain medical conditions should speak with a healthcare professional before starting L-citrulline supplementation.

Extra caution may be appropriate for people who:

• Have low blood pressure
• Have cardiovascular disease
• Take blood-pressure medication
• Take nitrate medication
• Take medication affecting vascular function
• Have kidney disease
• Are pregnant or breastfeeding
• Are scheduled for surgery
• Take multiple supplements that influence blood pressure or circulation

Because L-citrulline can influence the nitric oxide pathway and vascular function, combining L-citrulline with medications that lower blood pressure may require additional caution.

What Should You Look for in an L-Citrulline Supplement?

Not all citrulline supplements provide the same formulation.

When comparing products, consider the following:

1. Form of Citrulline

Check whether the product contains L-citrulline or citrulline malate.

2. Actual Citrulline Content

For citrulline malate, check how much actual L-citrulline the serving provides rather than looking only at the total citrulline-malate weight.

3. Serving Size

Compare the serving size with amounts investigated in clinical research.

4. Ingredient Transparency

A clear label should identify the active ingredients and quantities provided per serving.

5. Additional Ingredients

Some products combine L-citrulline with L-arginine, adaptogens, amino acids, or other ingredients.

Evaluate the complete ingredient list rather than focusing only on the headline ingredient.

6. Quality and Testing

Look for appropriate manufacturing and quality-control information when selecting a dietary supplement.

Is L-Citrulline Safe to Take Every Day?

Short-term clinical research suggests that L-citrulline is generally well tolerated.

However, long-term daily use has not been studied as extensively as short-term supplementation. Research has examined doses ranging from approximately 1.5 g per day to substantially higher amounts, depending on the study.

Higher research doses should not automatically be interpreted as recommended daily doses for general use.

People with medical conditions or people taking prescription medication should discuss daily L-citrulline supplementation with a healthcare professional.

Looking for an L-Citrulline and Nitric Oxide Formula?

L-citrulline is commonly used as a standalone supplement, but some formulations combine L-citrulline with L-arginine and other ingredients involved in broader performance and wellness routines.

Mars Nitric Oxide Capsules combine L-citrulline, L-citrulline malate, L-arginine, L-arginine HCl, and Arginine AKG, along with additional ingredients including Ashwagandha, Moringa, and Maca Root.

The combination is particularly relevant to the nitric oxide pathway discussed throughout this article, since both L-citrulline and L-arginine are involved in the body's nitric oxide production pathway.

The clinical research discussed in this article, however, examines specific L-citrulline preparations, doses, and study populations. Research findings should not automatically be applied to a multi-ingredient formulation.

For people looking to include L-citrulline and L-arginine alongside additional ingredients in a single daily wellness formula, Mars Nitric Oxide Capsules offer a convenient multi-ingredient option.

Frequently Asked Questions About L-Citrulline

What is L-citrulline?

L-citrulline is a nonessential amino acid naturally produced by the body and found in foods such as watermelon. L-citrulline can also be consumed as a dietary supplement.

What does L-citrulline do?

L-citrulline can be converted into L-arginine, which serves as a substrate for nitric oxide production. Nitric oxide plays a role in blood vessel relaxation and vascular function.

Does L-citrulline increase nitric oxide?

L-citrulline can increase circulating L-arginine, which provides substrate for nitric oxide production. However, the physiological response varies between individuals and research settings.

Does L-citrulline improve blood flow?

L-citrulline participates in the nitric oxide pathway involved in vascular relaxation. Some studies have reported improvements in vascular measures, although findings are not consistent across all populations.

Does L-citrulline lower blood pressure?

Research is mixed. Some systematic reviews and meta-analyses have reported reductions in systolic and diastolic blood pressure, while another meta-analysis found no significant overall effect. Original systematic review and meta-analysis Original systematic review and meta-analysis

Does L-citrulline improve exercise performance?

Some studies have reported improvements in resistance-training performance, while broader research has produced mixed findings. A 2022 meta-analysis found no significant overall improvement in aerobic performance, perceived exertion, VO₂ kinetics, or lactate. Original systematic review and meta-analysis

Does L-citrulline increase testosterone?

Current evidence does not establish L-citrulline as a reliable testosterone-support ingredient. Research has primarily focused on nitric oxide, vascular function, blood flow, and exercise-related outcomes.

How much L-citrulline should I take?

There is no universally established dose.

What is the difference between L-citrulline and citrulline malate?

L-citrulline is the amino acid by itself. Citrulline malate combines citrulline with malate. The amount of actual L-citrulline in citrulline malate depends on the formulation.

When should I take the L-citrulline?

Timing depends on the intended use. Sports nutrition studies commonly investigate L-citrulline or citrulline malate before exercise, while other studies use daily supplementation.

Is L-citrulline safe?

Short-term studies suggest that L-citrulline is generally well tolerated. However, long-term safety data remain more limited. People with medical conditions or people taking prescription medication should consult a healthcare professional before supplementation.

The Bottom Line

L-citrulline is a naturally occurring amino acid that plays an important role in the body's L-arginine and nitric oxide pathways.

Research has investigated L-citrulline for several areas, including vascular function, blood pressure, exercise performance, and erectile function. The strongest biological rationale for L-citrulline involves its conversion to L-arginine and the subsequent role of L-arginine in nitric oxide production.

Research involving blood pressure has produced mixed findings. Some systematic reviews have reported reductions in blood pressure, while other analyses have found no significant overall effect.

Exercise research is similarly mixed. Individual studies have reported improvements in resistance-training performance, while broader analyses have found limited evidence for improvements in aerobic performance.

L-citrulline has also been investigated for erectile function. A small clinical trial involving men with mild erectile dysfunction reported improvements in erection hardness following 1.5 g of L-citrulline daily for one month. However, the small sample size means the findings require confirmation through larger clinical trials.

L-citrulline and L-arginine are closely connected, but the two amino acids are processed differently by the body. L-citrulline can increase circulating L-arginine and is therefore commonly included in nitric oxide-focused formulations.

When choosing an L-citrulline supplement, consider the form, actual L-citrulline content, serving size, additional ingredients, quality standards, and intended use.

For people interested in combining L-citrulline and L-arginine in a broader daily wellness formula, Mars Nitric Oxide Capsules provide both amino acids alongside additional ingredients including Ashwagandha, Moringa, and Maca Root.

Most importantly, people with existing medical conditions or people taking prescription medication should speak with a healthcare professional before starting L-citrulline supplementation.

This article is for informational purposes only and does not provide medical advice. Dietary supplements are not intended to replace a balanced diet, prescribed medication, or professional medical care. People with a medical condition, people taking prescription medication, people who are pregnant or breastfeeding, or people considering L-citrulline for a specific health concern should consult a qualified healthcare professional before using dietary supplements.