CircO2 Review: Does This Nitric Oxide Lozenge Actually Work?

This article contains affiliate links. We only recommend products we have independently researched. Reviewed by the HealthyMag Editorial Team. Last updated: July 2026.

If you have spent any time reading about heart health, circulation, or “feeling 20 years younger,” you have probably been served an ad for CircO2. The pitch is seductive: a little lozenge that dissolves under your tongue and restores the nitric oxide your body stops making as you age. But does CircO2 actually work, or is it just clever marketing wrapped around a beet supplement? We dug into the ingredients, the real science, and what actual customers say — the good and the bad — to give you a straight answer.

What CircO2 Is

CircO2 is a nitric oxide (NO) support supplement made by Advanced Bionutritionals, a well-known US supplement company. It comes primarily as a fast-dissolving lozenge you place on your tongue (there is also a beet-based powder version). Each box typically contains 30 lozenges, a month’s supply, and the product is sold through the company’s own website via the Digistore24 checkout platform.

The core idea is straightforward: your body’s ability to produce nitric oxide — a signaling molecule that helps blood vessels relax and widen — declines with age. This is well documented; research on vascular aging shows that the capacity of the endothelium (the lining of your blood vessels) to generate bioactive nitric oxide falls as we get older, contributing to stiffer arteries and reduced circulation. CircO2 aims to top up NO through two routes: dietary nitrate from beet root, and the amino acid L-citrulline.

CircO2 is a dietary supplement, not a drug. It has not been evaluated by the FDA, and it is not intended to diagnose, treat, cure, or prevent any disease. That is not a knock on CircO2 specifically — it is true of every supplement on the market — but it is important context for the claims below.

How It Is Supposed to Work: The Oral Nitric Oxide Science

This is where CircO2 is more interesting than the average supplement, because the mechanism it relies on is real and has been studied seriously.

When you eat nitrate-rich foods (beets, leafy greens), the nitrate is absorbed, concentrated in your saliva, and then converted to nitrite by friendly bacteria living on your tongue. That nitrite is swallowed, converted to nitric oxide in the acidic stomach, and enters circulation. This is called the entero-salivary nitrate–nitrite–nitric oxide pathway, and it is one of the two main ways your body makes NO.

The proof that oral bacteria matter comes from an elegant experiment. In a 2013 study published in Free Radical Biology and Medicine, Kapil and colleagues had healthy volunteers use antiseptic (chlorhexidine) mouthwash twice daily for a week. Killing the nitrate-reducing bacteria on the tongue cut oral nitrite production by about 90% and plasma nitrite by roughly 25% — and systolic and diastolic blood pressure rose by 2 to 3.5 mm Hg as a result. In other words, disrupt the oral NO pathway and blood pressure climbs; support it and the reverse can happen. This is why CircO2 is a lozenge designed to dissolve in the mouth rather than a pill you swallow whole — the idea is to engage the salivary pathway directly.

So the underlying premise — that boosting the nitrate-to-NO pathway can affect circulation and blood pressure — is not marketing fiction; it is supported by published human research. The open question is always whether a specific product delivers enough of the right stuff to reproduce those effects.

The Ingredients, Examined One by One

CircO2’s formula is short, which is a good sign — it suggests the company is targeting a specific mechanism rather than throwing a “kitchen sink” of trendy powders at you. Here is each ingredient and what the evidence actually says.

• Beet root powder (nitrates): The star ingredient, and the one with the strongest evidence. In a 2015 randomized, double-blind, placebo-controlled trial in the journal Hypertension, Kapil and colleagues gave 68 people with high blood pressure daily dietary nitrate (as beetroot juice) for four weeks. Systolic blood pressure dropped by roughly 8 mm Hg versus placebo — a clinically meaningful change. Beet nitrate is the real engine behind nitric oxide supplements. (For more, see our deep dive on beetroot juice for blood pressure and endurance.)
• L-citrulline: This amino acid feeds the other NO pathway — the enzyme-driven L-arginine route. Counterintuitively, L-citrulline raises blood arginine levels more effectively than taking L-arginine itself, because arginine is largely destroyed in the gut before it reaches the bloodstream. A 2008 study by Schwedhelm and colleagues in the British Journal of Clinical Pharmacology showed oral L-citrulline dose-dependently increased plasma arginine and improved NO-dependent signaling. So its inclusion is scientifically justified, though the dose in CircO2 is not disclosed in gram amounts, which makes it hard to know if it matches the levels used in studies (often 3g+).
• Hawthorn berry: A traditional heart tonic with some evidence for supporting circulation and mild blood-pressure effects, though the data is weaker and more mixed than for nitrate. Reasonable as a supporting ingredient, not a proven headline actor.
• Vitamin C: An antioxidant that helps protect nitric oxide from being broken down by free radicals, potentially extending how long NO stays active. A sensible, evidence-consistent addition.
• Vitamin B12: Included largely to support the conversion steps and general cardiovascular/nerve health. Useful especially for older adults, who are more likely to be low in B12, but not a direct NO booster.
• Magnesium: Involved in vascular relaxation and blood pressure regulation; a deficiency is common and worth correcting. A logical, low-risk inclusion.

Bottom line on ingredients: there is no snake oil here. Every component has a plausible rationale, and the two headliners — beet nitrate and L-citrulline — have genuine human evidence. The main weakness is transparency: CircO2 does not publish the exact milligram/gram dose of each ingredient, so you cannot verify whether the amounts match the studied doses.

What the Company Claims vs What Is Independently Proven

This is the most important section for anyone deciding whether to buy, so we will be blunt.

Advanced Bionutritionals markets CircO2 with striking numbers: nitric oxide “up 200% in 20 minutes” and blood flow “up 34%.” Here is the crucial caveat the ads do not emphasize: these figures come from the manufacturer’s own testing, not from independent, peer-reviewed clinical trials. That does not automatically make them false — but company-run testing on its own product is not the same standard of evidence as an independent randomized controlled trial, and it should be read as promotional data, not settled proof.

What is independently proven is the underlying category: dietary nitrate lowers blood pressure (Kapil 2015), the oral bacteria pathway is real (Kapil 2013), and L-citrulline raises NO precursors (Schwedhelm 2008). What is not independently verified is that CircO2 specifically produces the exact 200%/34% results. The honest framing: the science supports the ingredients; the specific product claims rest on the company’s own numbers.

CircO2 marketing claims vs the evidence

What Real Customers Say (3.9/5): Pros and Complaints

We looked at the aggregate customer feedback rather than cherry-picking testimonials. Across 1,618 reviews, CircO2 holds a 3.9 out of 5 star rating, with about 74% of customers saying they would recommend it. That is a respectable but not spectacular score — and it is far more honest than the “life-changing” tone of the sales page.

The math matters: 74% recommending means roughly 1 in 4 customers (about 26%) would not recommend it. A balanced review has to sit with that.

What happy customers report:

• More energy and stamina, especially during exercise or daily activity.
• Better circulation and warmer hands and feet.
• Some report modest improvements in blood pressure readings (though this should always be tracked with your doctor).
• Convenience — a fast-dissolving lozenge is easier than choking down big capsules.

What critical reviewers report:

• “It didn’t work for me.” A meaningful share of users felt no noticeable difference. Individual response to nitrate varies a lot, partly because it depends on your own oral bacteria.
• Price complaints. At around $42–$50 per box, ongoing use adds up, and this is the single most common gripe.
• Taste. Some dislike the flavor of the dissolving lozenge.
• Auto-ship / subscription friction. A few reviewers mention needing to manage recurring orders carefully.

Our read: a 3.9/5 from over 1,600 people is the signature of a real product that genuinely helps a majority of users but is not a miracle for everyone. That is exactly what you would predict from the science, since nitrate response is individual.

Side Effects and Safety

Is CircO2 safe? For most healthy adults, the ingredients are generally well tolerated, and nitrate from vegetables has a long safety record. Reported side effects are usually mild:

• Lightheadedness or headache — because nitric oxide dilates blood vessels, some people feel a mild drop in blood pressure, especially early on.
• Beeturia — harmless pink/red discoloration of urine or stool from beet pigment.
• Mild digestive upset in some users.

Important safety notes: if you take blood pressure medication, nitrates (like nitroglycerin), or erectile-dysfunction drugs (PDE5 inhibitors such as sildenafil), combining them with a nitric oxide booster could lower your blood pressure too much. The same caution applies if you are pregnant, nursing, or have kidney disease. CircO2 is not a substitute for prescribed blood pressure medication, and you should not stop or adjust any prescription based on a supplement. Talk to your doctor or pharmacist before starting — this is genuinely important, not boilerplate. If you are on statins, our article on CoQ10 for heart health and statin users covers related supplement interactions worth understanding.

Who It Is For — and Who Should Skip It

CircO2 may be a reasonable try if you: are middle-aged or older, want circulation/energy support, eat few nitrate-rich vegetables, and are looking for a convenient way to support the NO pathway with an ingredient list backed by real science.

You should probably skip it (or clear it with a doctor first) if you: already have low blood pressure, take BP meds, nitrates, or ED drugs, are pregnant or nursing, have kidney disease, or expect a supplement to replace medication or a healthy diet. It is also not the pick if dose transparency is a dealbreaker for you, since exact amounts are not disclosed.

Price, Guarantee, and Where to Buy

CircO2 is sold directly through Advanced Bionutritionals (checkout via Digistore24), not on Amazon or in stores — buying from the official source is the only way to be sure you get an authentic product and the guarantee. Typical pricing:

• 1 box: about $49.95
• 3 boxes: about $44.95 per box
• 6 boxes: about $41.50 per box

One disclosure worth flagging: CircO2 is often endorsed by Dr. Greg Eckel, a Naturopathic Physician — who is compensated by the company. A paid endorsement is not disqualifying, but it is not independent, and you should weigh it as marketing rather than a neutral medical recommendation.

CircO2 vs Neo40: An Honest Comparison

Neo40 (by HumanN) is the other big name in nitric-oxide lozenges, so a comparison is fair. Both are lozenge-format NO boosters built on beet-derived nitrate and the same salivary pathway; the two products are more alike than different.

Honest take: neither is clearly superior — they are close cousins. CircO2’s edge is its long 90-day guarantee and a slightly broader supporting formula. Neo40’s edge is wider availability and, in some kits, a saliva NO test strip so you can gauge whether you are responding. If the guarantee matters most to you, CircO2 wins; if you want to buy from Amazon or physically measure your NO levels, Neo40 has the advantage. Both share the same core caveat: individual response varies.

Frequently Asked Questions

Does CircO2 really work?

It works for many people but not everyone. The science behind its ingredients — beet nitrate and L-citrulline raising nitric oxide — is well supported by independent research. In real-world reviews, about 74% of 1,618 customers recommend it (3.9/5 stars), while roughly a quarter saw no clear benefit. Response to nitrate is individual, partly because it depends on your own oral bacteria.

Is CircO2 safe?

For most healthy adults, yes — the ingredients are vegetable-derived and generally well tolerated, with mild possible side effects like lightheadedness, headache, or harmless red-tinted urine. It has not been evaluated by the FDA. If you take blood pressure medication, nitrates, or ED drugs, or are pregnant, nursing, or have kidney disease, talk to your doctor first.

Is CircO2 a scam?

No. A scam sells nothing of value; CircO2 is a real product from an established company, with a legitimate ingredient list, a real 3.9/5 customer rating, and a 90-day money-back guarantee. The fair criticism is not fraud but over-promising: its headline stats (“nitric oxide up 200%”) come from the company’s own testing, and it uses a paid physician endorsement. Manage your expectations and it is a legitimate purchase.

What are the side effects of CircO2?

The most commonly reported side effects are mild: temporary lightheadedness or headache (from blood-vessel dilation), harmless pink discoloration of urine or stool from beet pigment (beeturia), and occasional mild digestive upset. Serious side effects are uncommon, but combining it with BP or ED medication can lower blood pressure too much.

How do you take CircO2?

You place one lozenge on your tongue and let it dissolve slowly rather than swallowing it whole — this is deliberate, so the nitrate engages the bacteria in your mouth that kick off the nitric oxide pathway. Most people take it once daily. Follow the label and your doctor’s guidance.

Is CircO2 safe to take with blood pressure medication?

Only with your doctor’s approval. Because CircO2 is designed to boost nitric oxide and relax blood vessels, stacking it on top of blood pressure drugs, prescription nitrates, or ED medications can drop your blood pressure too far. It is a supplement, not a replacement for prescribed medication — never stop or change a prescription on your own.

Where can you buy CircO2?

CircO2 vs Neo40 — which is better?

They are very similar nitric-oxide lozenges built on the same beet-nitrate science. CircO2 offers a longer 90-day guarantee and a slightly broader supporting formula; Neo40 offers wider retail availability and, in some kits, a saliva test strip to measure your NO response. Neither is decisively better — pick based on whether the guarantee or the measurability matters more to you.

The Bottom Line

CircO2 is a legitimate, science-grounded nitric oxide supplement — not a scam, but not a miracle either. Its strengths are real: the beet-nitrate and L-citrulline mechanisms are backed by independent human research, the formula is clean and rationally designed, and the 90-day money-back guarantee genuinely de-risks trying it. Its weaknesses are equally real: the flashiest marketing numbers come from the company’s own testing rather than independent trials, the physician endorsement is paid, doses are not fully disclosed, the price draws frequent complaints, and its honest 3.9/5 rating means roughly 1 in 4 users are not impressed.

Our verdict: if you are an older adult wanting circulation and energy support, eat few vegetables, and go in with realistic expectations, CircO2 is a reasonable, refund-protected experiment — best used alongside, never instead of, a healthy diet and any medication your doctor has prescribed. Track your own results (and your blood pressure) and lean on that 90-day guarantee if it does not deliver for you.

Sources

1. Kapil V, Haydar SMA, Pearl V, Lundberg JO, Weitzberg E, Ahluwalia A. “Physiological role for nitrate-reducing oral bacteria in blood pressure control,” Free Radical Biology and Medicine, 2013;55:93–100. https://pmc.ncbi.nlm.nih.gov/articles/PMC3605573/
2. Kapil V, Khambata RS, Robertson A, Caulfield MJ, Ahluwalia A. “Dietary Nitrate Provides Sustained Blood Pressure Lowering in Hypertensive Patients: A Randomized, Phase 2, Double-Blind, Placebo-Controlled Study,” Hypertension, 2015;65(2):320–327. https://www.ahajournals.org/doi/10.1161/hypertensionaha.114.04675
3. Schwedhelm E, Maas R, Freese R, et al. “Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism,” British Journal of Clinical Pharmacology, 2008;65(1):51–59. https://pmc.ncbi.nlm.nih.gov/articles/PMC2291275/
4. Seals DR, Jablonski KL, Donato AJ. “Vascular endothelial dysfunction with aging: endothelin-1 and endothelial nitric oxide synthase,” American Journal of Physiology-Heart and Circulatory Physiology, 2009. https://pmc.ncbi.nlm.nih.gov/articles/PMC2711733/
5. Rosier BT, et al. “Pathways Linking Oral Bacteria, Nitric Oxide Metabolism, and Health,” PMC (National Library of Medicine), 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9124908/