“Nitric oxide” can mean a molecule your body makes, a supplement you buy, or a medical gas used in hospitals — and the side effects are completely different.
Nitric oxide is a signaling molecule your blood vessels use to relax and widen. That is why “nitric oxide boosters” are sold for workout pumps, blood pressure, and circulation. But when people search for nitric oxide side effects, they are often reading about two very different things mixed together: supplements that help the body make its own nitric oxide, and nitric oxide gas, a medical treatment and workplace hazard.
This guide separates them. Supplements such as L-arginine, L-citrulline, and beetroot do not contain nitric oxide at all, and their risks are mostly about blood pressure and drug interactions. Inhaled nitric oxide gas is a hospital treatment with its own serious side effects. Here is what the research says about each.
Key Takeaways
- “Nitric oxide supplements” usually contain L-arginine, L-citrulline, or dietary nitrate (often from beetroot), which the body uses to make nitric oxide (1, 2).
- Their main effect is lower blood pressure, which is also their main risk for people already on blood pressure or heart medication (2, 6).
- L-arginine should not be used after a heart attack. A trial was stopped early after more deaths in the L-arginine group (3).
- Nitrate heart medicines combined with erectile dysfunction drugs can cause a dangerous drop in blood pressure (4, 5).
- Inhaled nitric oxide gas is a hospital treatment; its side effects include methemoglobinemia and rebound pulmonary hypertension (10, 11).
- For workouts, the performance benefit of nitric oxide boosters is small (8).
What “Nitric Oxide Supplements” Actually Contain
Nitric oxide is a gas that lasts only seconds in the body, so it cannot be put in a capsule. Supplements instead supply raw materials for making it. L-arginine is used directly by the enzyme nitric oxide synthase, and L-citrulline is converted into L-arginine first. Beetroot juice and other nitrate-rich foods work through a separate pathway: nitrate is converted to nitrite, which can then be turned into nitric oxide (1, 2).
A review of these supplements notes that they can cause mild to moderate side effects, that smaller or divided doses may reduce some of them, and that because they can worsen certain health conditions and interfere with medicines, they should be used under medical supervision (1).
Some pre-workout products sold as nitric oxide boosters also contain other active ingredients, such as caffeine. Read the full ingredient label, not just the front of the tub.
L-Arginine Side Effects
L-arginine is the most familiar nitric oxide supplement and the one with the most important warning.
- Side effects such as digestive upset. A review of nitric oxide supplements notes they can cause mild to moderate side effects, and that smaller or divided doses may avoid some of them (1).
- Lower blood pressure. Because nitric oxide widens blood vessels, L-arginine can add to the effect of blood pressure medicines.
- Not after a heart attack. In the VINTAGE MI randomized trial, 153 people who had recently had a heart attack took L-arginine (target dose 3 grams three times a day) or placebo alongside standard treatment. L-arginine did not improve blood vessel stiffness or heart function, and 6 people in the L-arginine group died during the six months compared with none on placebo. The safety committee closed the trial, and the authors concluded L-arginine should not be recommended after a heart attack (3).
L-Citrulline Side Effects
L-citrulline raises L-arginine levels in the blood and is often used in “pump” products. In a randomized trial of 25 postmenopausal women with high blood pressure, 10 grams of L-citrulline a day for four weeks raised L-arginine levels, improved blood vessel function, and lowered aortic blood pressure compared with placebo (6).
That blood pressure effect is useful for some people and a potential problem for others. If you already take medication for high blood pressure, or tend to have low blood pressure, check with your doctor before using it.
Beetroot and Dietary Nitrate Side Effects
SaveBeetroot juice and nitrate-rich vegetables raise nitrite levels in the blood and lower resting blood pressure (2). Side effects are usually minor:
- Pink or red urine (beeturia). This affects about 10 to 14 percent of people after eating beets and is more common in people with iron deficiency (7). It is harmless, but red urine without beetroot needs a doctor’s attention.
- Mild side effects in some people, which smaller or divided doses may reduce (1).
- Lower blood pressure, which matters for people on blood pressure medication (2).
As for performance, a meta-analysis of 118 studies found that nitrate-rich foods such as beetroot provided only trivial improvements in endurance, with no effect seen in women or highly trained athletes, and no benefit from L-citrulline-rich foods (8).
The Interaction You Must Know: ED Drugs and Nitrates
Nitrate medicines such as nitroglycerin are used for angina because they release nitric oxide and widen blood vessels (14). Erectile dysfunction drugs such as sildenafil work on the same pathway. Taken together, they can cause a sudden, dangerous drop in blood pressure. Nitrates should not be given within 24 hours of sildenafil (4), and acute nitrate treatment should be avoided after recent use of these drugs (5).
Nitric oxide supplements are not nitrate medicines, but they push in the same direction by lowering blood pressure. If you take erectile dysfunction medicine, nitrates, or blood pressure drugs, talk to your doctor or pharmacist before adding a nitric oxide booster.
Side Effects of Medical Nitric Oxide Gas
SaveInhaled nitric oxide is a prescription gas given in hospitals, most often to newborns and adults with pulmonary hypertension or severe breathing problems. It is delivered through specialized equipment with continuous monitoring and is not something used at home (11). Its known risks include:
- Methemoglobinemia. Nitric oxide can convert hemoglobin into a form that cannot carry oxygen, which can turn the lips and skin bluish (10).
- Rebound pulmonary hypertension. Stopping inhaled nitric oxide after prolonged use can cause blood pressure in the lungs to spike, which is why it is weaned carefully (11, 12).
- Pulmonary edema in some patients. Fluid in the lungs developed during nitric oxide testing in two patients with pulmonary hypertension linked to CREST syndrome (13).
Outside medicine, nitric oxide is an industrial and air pollution gas. U.S. workplace exposure limits are 25 ppm averaged over a work shift, and 100 ppm is considered immediately dangerous to life and health. Exposure can irritate the eyes, skin, nose, and throat and cause drowsiness, unconsciousness, and methemoglobinemia (9). None of this applies to swallowing an L-arginine capsule — which is why mixing the two topics causes so much confusion.
Your Body’s Own Nitric Oxide
The nitric oxide your blood vessel lining makes is essential. Impaired nitric oxide activity is linked to arterial stiffness and high blood pressure (15). For most people, the simplest way to support it is through food rather than supplements: leafy greens, beetroot, and other nitrate-rich vegetables raise nitrite levels naturally (2), and they come with fiber and other nutrients. A plant-forward way of eating or simply a balanced everyday diet makes those vegetables easy to include. If your interest is in circulation and breathing, see evidence-based ways to support blood oxygen levels.
Who Should Avoid Nitric Oxide Supplements
- Anyone who has recently had a heart attack (L-arginine especially) (3).
- People taking nitrate medicines or erectile dysfunction drugs, unless a doctor has approved it (4, 5).
- People with low blood pressure or who take blood pressure medication (2, 6).
- People with heart, kidney, or liver disease, or who are pregnant or breastfeeding, without medical advice (1).
Frequently Asked Questions
Are nitric oxide supplements safe?
For most healthy adults, food-based nitrate (like beetroot) and supplements such as L-citrulline appear to cause only mild side effects. The risks are specific: they can lower blood pressure, they interact dangerously with erectile dysfunction drugs if combined with nitrate medicines, and L-arginine should not be used after a heart attack. Check with your doctor first if you have heart disease, low blood pressure, or take regular medication.
Can I take a nitric oxide booster with Viagra or Cialis?
Talk to your doctor first. Nitrate medicines such as nitroglycerin must not be taken within 24 hours of sildenafil because the combination can cause a dangerous drop in blood pressure. Nitric oxide boosters are not the same as nitrate drugs, but they also work by widening blood vessels and can lower blood pressure, so combining them with erectile dysfunction medicine is not something to try on your own.
Why is my urine pink or red after beetroot?
That is beeturia, and it is usually harmless. It happens in roughly 10 to 14 percent of people after eating beets and is more common in people with low iron. If your urine is red and you have not eaten beetroot, see a doctor, since blood in the urine needs checking.
What does 20 ppm nitric oxide mean?
Parts per million (ppm) is a measure of nitric oxide gas in the air someone breathes. It is used for medical inhaled nitric oxide in hospitals and for workplace exposure limits, which are 25 ppm averaged over a work shift. It has nothing to do with supplement doses, which are measured in grams of L-arginine, L-citrulline, or nitrate.
Do nitric oxide boosters improve workouts?
The benefit is small. A large meta-analysis found that nitrate-rich foods such as beetroot gave trivial improvements in endurance performance, with no benefit seen in women or highly trained athletes, and foods rich in L-citrulline showed no effect.
The Bottom Line
Nitric oxide side effects depend on what you mean by nitric oxide. Booster supplements such as L-arginine, L-citrulline, and beetroot do not contain the gas; their main effects and risks involve lower blood pressure, digestive upset, and interactions with heart and erectile dysfunction medicines, and L-arginine should not be taken after a heart attack. Inhaled nitric oxide is a hospital treatment with serious but well-managed risks such as methemoglobinemia and rebound pulmonary hypertension. For most people, nitrate-rich vegetables are the safest way to support nitric oxide — and anyone with a heart condition or on regular medication should speak to a doctor before using a booster.
This article is for informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting any supplement, particularly if you have an existing health condition or take medication.
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