Do NOx Supplements Work? An Evidence-Based Guide

December 15, 2025 •

4 min read

Nitric oxide supplements, often containing precursors like L-arginine and L-citrulline, do not contain actual nitric oxide gas but are intended to boost the body's natural production. Athletes and fitness enthusiasts widely use these products to enhance performance, but the question remains: do NOx supplements work as claimed?

Quick Summary

Nitric oxide supplements are formulated with precursor ingredients to support the body's NO production, aiming to improve athletic performance, blood flow, and recovery. Research on their efficacy is mixed, with some studies showing benefits for moderately trained individuals, while others find no significant effect, especially in elite athletes. Safety concerns and side effects are also important to consider.

Key Points

Precursors, Not NO Gas: NOx supplements contain ingredients like L-citrulline, L-arginine, and nitrates, which are precursors that the body converts into nitric oxide.
Mixed Efficacy: The research is mixed; benefits for athletic performance, blood pressure, and ED have been noted, but not all studies show positive results.
Citrulline More Effective: L-citrulline is often more effective than L-arginine at boosting nitric oxide because it has better bioavailability.
Dietary Nitrates Work: Nitrate-rich foods like beetroot and spinach are proven, effective ways to increase the body's nitric oxide levels naturally.
Side Effects and Risks: Side effects can include GI issues and headaches, and those with heart conditions or low blood pressure should consult a doctor due to potential risks.
Benefit Varies by Training Level: Benefits for exercise performance are more evident in untrained or moderately trained individuals than in highly trained athletes.

Understanding Nitric Oxide and Supplementation

Nitric oxide (NO) is a molecule that plays a crucial role in the body, primarily by acting as a vasodilator—relaxing and widening blood vessels to improve blood flow. Enhanced blood flow ensures that muscles and organs receive more oxygen and nutrients during exercise, leading to the potential benefits often associated with NO supplementation. Unlike inhaled medical-grade nitric oxide gas, dietary NOx supplements contain precursor ingredients that the body can convert into nitric oxide. The most common precursors found in these supplements are the amino acids L-arginine and L-citrulline, along with nitrates derived from foods like beetroot.

The two primary pathways for the body to create nitric oxide are:

The L-arginine pathway, where the enzyme nitric oxide synthase (NOS) converts L-arginine into NO.
The nitrate-nitrite-NO pathway, which is oxygen-independent and converts dietary nitrates into NO.

The Science on Whether NOx Supplements Work

The effectiveness of NOx supplements is a complex and often debated topic within the fitness and medical communities, with study results being quite mixed. The outcome seems highly dependent on the supplement's ingredients, dosage, and the individual's training level.

Evidence for L-arginine has been particularly inconsistent. This is because a significant amount of ingested L-arginine is metabolized before it can be effectively used to produce nitric oxide, limiting its bioavailability. Conversely, L-citrulline has shown more promising results. It bypasses metabolism in the liver and is converted to L-arginine in the kidneys, leading to more sustained increases in plasma arginine and subsequently higher NO levels.

Dietary nitrates, often from beetroot, have also demonstrated effectiveness, with several studies noting improvements in exercise performance and endurance. This is because nitrates are converted to nitric oxide through the NOS-independent pathway, which is particularly active when oxygen levels are low during intense exercise. Combining citrulline with nitrates may offer a synergistic effect, potentially leading to greater endurance benefits.


Ingredient	Bioavailability	Performance Evidence	Side Effects	Key Takeaway
L-Arginine	Poor; significant first-pass metabolism.	Mixed results, often showing little to no effect in studies.	Gastrointestinal distress at high doses.	Not the most reliable precursor for increasing NO levels.
L-Citrulline	High; bypasses liver metabolism.	More consistent evidence for improved performance and reduced muscle soreness.	Generally well-tolerated, some stomach discomfort at high doses.	Superior to L-arginine for boosting NO levels effectively.
Beetroot/Nitrates	Good; uses a separate metabolic pathway.	Strong evidence for increased endurance and reduced oxygen cost of exercise.	Beeturia (harmless reddish urine/stool).	A natural, effective way to increase nitric oxide.

Potential Benefits and Side Effects

While the primary interest in NOx supplements is athletic performance, research has explored other potential health benefits related to improved blood flow:

Potential Benefits

Enhanced Exercise Performance: Increased oxygen and nutrient delivery to muscles may lead to greater endurance and reduced fatigue. This is most pronounced in untrained or moderately trained individuals, with less effect on elite athletes who already have efficient NO pathways.
Improved Cardiovascular Health: By relaxing blood vessels, NO can help lower blood pressure. Some studies suggest benefits, particularly for individuals with pre-existing conditions like high blood pressure.
Reduced Erectile Dysfunction: The vasodilation effect of nitric oxide can improve blood flow to the genitals, which can be beneficial for men with mild-to-moderate ED.
Faster Muscle Recovery: Increased blood flow can help clear metabolic waste and deliver nutrients for muscle repair after strenuous exercise, potentially reducing delayed-onset muscle soreness.

Potential Side Effects

Though generally considered safe for most healthy adults within recommended dosages, NOx supplements can have side effects, particularly with higher intake:

Gastrointestinal Issues: Bloating, nausea, diarrhea, and stomach pain are among the most commonly reported side effects.
Lowered Blood Pressure: The vasodilating effect can cause blood pressure to drop, potentially leading to dizziness or fainting, especially in individuals with naturally low blood pressure.
Headaches and Palpitations: Some users report experiencing headaches and heart palpitations.

Who Should Avoid NOx Supplements?

Certain individuals should exercise caution or avoid NOx supplements entirely due to potential risks:

People with liver disease or low blood pressure.
Anyone who has recently had a heart attack.
Individuals with guanidinoacetate methyltransferase (GAMT) deficiency.
Those taking certain medications, such as blood pressure drugs or erectile dysfunction medication, due to potential interactions.

Natural Alternatives to NOx Supplements

For many, increasing nitric oxide naturally through diet and lifestyle may be a safer and more reliable option than supplementation. Foods rich in nitrates and specific amino acids are excellent sources:

Nitrate-Rich Vegetables: Beets, spinach, kale, and arugula are packed with nitrates that your body converts to NO.
Antioxidant-Rich Fruits: Oranges, berries, and pomegranates contain flavonoids and vitamin C, which help protect and extend the life of nitric oxide in the body.
Lifestyle Choices: Regular exercise stimulates the production of nitric oxide, and practicing nasal breathing can also help increase levels.

Conclusion

In conclusion, whether NOx supplements work depends heavily on the specific ingredients and the individual. While supplements containing L-citrulline and nitrates, particularly from beetroot, show more consistent evidence for improving exercise performance, supplements based solely on L-arginine are less reliable due to poor bioavailability. The benefits are most likely to be noticed by untrained or moderately trained individuals rather than elite athletes. It's crucial to be aware of potential side effects and consider dietary changes and lifestyle improvements as safer, reliable methods for boosting nitric oxide levels naturally. Anyone with pre-existing health conditions or on medication should consult a healthcare provider before starting supplementation to avoid serious risks.

For more detailed research and potential applications of dietary supplements in improving nitric-oxide synthesis, see this Journal of Preventive Medicine and Hygiene article.

Frequently Asked Questions

NOx supplements are dietary products that contain precursor ingredients, such as L-arginine, L-citrulline, and nitrates from sources like beetroot, which the body uses to create nitric oxide.

By providing precursors for nitric oxide, these supplements can help widen blood vessels, increasing blood flow to muscles. This improved circulation can lead to better oxygen and nutrient delivery, potentially enhancing endurance and reducing muscle soreness.

For most healthy adults, NOx supplements are generally considered safe when taken within recommended dosages. However, side effects like stomach discomfort and headaches are possible, and individuals with certain health conditions or on specific medications should consult a doctor first.

L-citrulline is often considered more effective than L-arginine for boosting nitric oxide levels. This is because L-citrulline has better bioavailability and bypasses initial metabolism, leading to more sustained increases in the body's arginine supply.

Yes, you can increase your body's nitric oxide levels naturally through diet and lifestyle. Eating nitrate-rich vegetables like beets and spinach, and engaging in regular exercise are effective, science-backed methods.

No, the effectiveness of NOx supplements varies by individual. Factors like genetics, diet, and training level play a role. The benefits for exercise performance are typically more noticeable in untrained or moderately trained individuals compared to elite athletes.

Yes, they can interact with certain medications. It is especially important to consult a healthcare provider if you are taking drugs for blood pressure or erectile dysfunction, as NOx supplements can amplify their effects.