Nitric oxide is one of those molecules that does enormous work silently. It relaxes blood vessels, supports healthy blood pressure, helps deliver oxygen to your muscles and organs, and plays a role in cellular energy production. The problem: your body's ability to produce it naturally peaks in your 20s and declines steadily from there. By the time you hit your 40s and 50s, the decline is significant enough to affect how you feel day to day — and most people never connect the dots.
What Happens When Nitric Oxide Drops
Nitric oxide isn't just one function — it's a signaling molecule involved in multiple systems. When production slows down, the effects can show up in ways that are easy to attribute to "just getting older."
- ⚠ Blood vessels become less flexible, making it harder for the cardiovascular system to maintain healthy circulation
- ⚠ Oxygen delivery to muscles and organs slows, which can manifest as lower stamina and sluggishness
- ⚠ The endothelium (inner lining of blood vessels) becomes less responsive, contributing to vascular stiffness over time
- ⚠ Exercise recovery takes longer because nutrient and oxygen delivery to working muscles is less efficient
- ⚠ Cellular energy production in mitochondria is partially dependent on adequate NO signaling
Our take: The age-related decline in nitric oxide production is well-documented in cardiovascular research. It's a real physiological change, not a marketing scare tactic. The practical question is whether supplementation can meaningfully support NO levels once your body's own production has slowed — and that depends on the specific ingredients and how they're delivered.
How Beetroot & L-Citrulline Support NO Production
Before evaluating CircO2 specifically, let's look at what the published research says about its two primary NO-boosting ingredients.
Beetroot: The Nitrate Pathway
Beetroot is naturally rich in dietary nitrates. When consumed, oral bacteria convert these nitrates to nitrites, which are then converted to nitric oxide in the body. Multiple studies have shown beetroot supplementation supports healthy blood flow and exercise performance in older adults.
L-Citrulline: The Recycling Pathway
L-Citrulline is an amino acid that your body converts to L-arginine, which is then used to produce nitric oxide via the eNOS enzyme. Research suggests L-citrulline may be more effective than taking L-arginine directly because it bypasses first-pass liver metabolism.
Two Pathways, One Goal
Your body produces NO through two main routes: the L-arginine/eNOS pathway and the nitrate-nitrite-NO pathway. Combining beetroot (nitrate pathway) with L-citrulline (L-arginine pathway) addresses both mechanisms simultaneously, which is a sensible approach from a biochemistry standpoint.
What the Research Shows
Published studies on beetroot and L-citrulline demonstrate measurable improvements in blood flow markers, exercise tolerance, and vascular flexibility. The research is strongest for populations over 40 where endogenous NO production has declined and supplementation can fill the gap.
Timing and Consistency Matter
Nitric oxide has a very short half-life in the body — just seconds. This means consistent daily supplementation is more relevant than occasional use. The research showing benefits typically involves daily intake over weeks to months, not one-time doses.
The Oral Bacteria Factor
A key finding in NO research: the nitrate-to-nitrite conversion depends heavily on bacteria in your mouth. Mouthwash that kills oral bacteria can actually reduce nitric oxide production. This is why the delivery method — and specifically whether it engages the oral cavity — matters more than many people realize.
What's Inside CircO2
CircO2 is a quick-dissolving lozenge designed to be placed in the mouth and allowed to dissolve. Each box contains 30 lozenges (a 30-day supply at one per day). Here are the core ingredients and their roles.
L-Citrulline
An amino acid your body converts to L-arginine, which is a direct precursor to nitric oxide via the eNOS enzyme. Research suggests L-citrulline is actually better absorbed than L-arginine itself because it avoids breakdown in the liver during first-pass metabolism.
Beetroot Extract
A concentrated source of dietary nitrates. Beetroot has become one of the most studied natural NO-boosting compounds, with research showing its effects on blood flow, exercise endurance, and vascular health. The extract form provides a standardized nitrate dose.
Supporting Nutrients
The formula includes additional vitamins and minerals that play roles in cardiovascular function and NO metabolism. These supporting nutrients are included to create a more complete environment for nitric oxide production rather than relying on a single ingredient.
What's NOT in It
CircO2 is certified vegan, non-GMO, gluten-free, dairy-free, and soy-free. No artificial preservatives or synthetic fillers. For a lozenge that you dissolve in your mouth, a clean ingredient profile is particularly important since it has direct mucosal contact.
Our take: The two core ingredients — L-citrulline and beetroot — are among the most well-researched natural nitric oxide boosters available. The formula addresses both major NO production pathways, which is a sound approach. The clean certifications (vegan, non-GMO, allergen-free) are a plus, especially for a lozenge format that dissolves in the mouth.
The Lozenge Question: Does Format Actually Matter?
Most nitric oxide supplements come as capsules or powders. CircO2 uses a quick-dissolving lozenge. Is this a gimmick or a genuine advantage?
Key insight: The oral bacteria that convert nitrates to nitrites live in your mouth, not your stomach. A lozenge that dissolves slowly in the mouth gives these bacteria extended contact time with the beetroot nitrates. Capsules skip this step entirely by delivering everything to the stomach. This isn't theoretical — the nitrate-nitrite-NO pathway via oral bacteria is well-established in published research. The lozenge format is a scientifically rational design choice, not a marketing gimmick.
Honest Assessment: What This Can and Cannot Do
No supplement analysis is complete without drawing clear lines between what is supported and what is not. Here's our assessment of CircO2.
What the science supports:
- • Beetroot and L-citrulline are among the most researched natural NO precursors with published clinical data
- • Nitric oxide production does decline with age, and this decline affects cardiovascular and circulatory function
- • The lozenge delivery format has a sound scientific basis for engaging the oral nitrate-nitrite conversion pathway
- • Combining two different NO pathways (nitrate + L-arginine) in one formula is a rational, evidence-informed approach
- • Daily NO supplementation has shown measurable effects on blood flow markers in clinical studies
What to be realistic about:
- • NO supplements support circulation but cannot replace cardiovascular exercise, a healthy diet, or medical treatment
- • Individual responses vary significantly — genetics, diet, existing NO levels, and medications all influence outcomes
- • Anyone on blood pressure medication or blood thinners must consult their doctor before taking NO supplements
- • Measurable benefits typically require consistent daily use over weeks, not a single dose
- • A one-month supply at full price ($49.95) makes it a premium-priced supplement in this category
Bottom line: CircO2 is a thoughtfully designed nitric oxide supplement that makes smart use of the lozenge format. The ingredient selection is grounded in published research, the delivery method has scientific merit, and the formula addresses both major NO production pathways. It won't replace exercise or a healthy lifestyle — but for people over 40 who want to support their body's declining NO production, the formulation logic is sound.
Who Should Consider CircO2
Based on the formulation, the delivery format, and the science behind nitric oxide supplementation, here's our assessment of fit.
Good fit if you:
- ● Are over 40 and concerned about age-related circulation changes
- ● Want to support healthy blood pressure through natural NO precursors
- ● Notice lower energy or stamina that may be linked to reduced blood flow
- ● Prefer a lozenge format over swallowing capsules
- ● Want a supplement backed by ingredients with published clinical research
- ● Are willing to use consistently for 60-90 days to properly evaluate
Not the right fit if you:
- ● Are looking for a substitute for cardiovascular exercise or a healthy diet
- ● Take blood pressure medication or blood thinners without consulting your doctor first
- ● Expect overnight or dramatic changes from a single lozenge
- ● Are under 30 with presumably healthy NO production
- ● Are pregnant or nursing (consult your healthcare provider first)
Who's Behind the Formula
CircO2 is made by Advanced Bionutritionals and formulated by Dr. Frank Shallenberger. Here's what we verified about the credentials behind this product.
Dr. Frank Shallenberger, MD
Board-certified in anti-aging medicine with over 44 years of clinical experience. Editor-in-Chief of the Second Opinion Newsletter. Dr. Shallenberger has formulated multiple products for Advanced Bionutritionals, including their Advanced Amino Formula. His clinical focus includes metabolic efficiency and cardiovascular support.
Advanced Bionutritionals
An established supplement company with a broad product line focused on health solutions for adults over 40. They manufacture in the USA and have been in business for over a decade. Products are available on their website, Amazon, and through authorized partners.
Product Standards
Vegan, non-GMO, gluten-free, dairy-free, and soy-free. Manufactured in the USA. Each box contains 30 individually dosed lozenges. The lozenge format requires careful formulation to ensure proper dissolution rate and taste while maintaining ingredient efficacy.
Our take: The formulator has legitimate medical credentials and decades of clinical experience. The company has an established track record across multiple products. For a supplement that directly affects cardiovascular function, having a physician-formulated product from a known manufacturer provides a meaningful layer of accountability.
Pricing & Packages
CircO2 is available in three package sizes. Each box contains 30 quick-dissolving lozenges (a 30-day supply at one lozenge per day).
1 BOX
$49.95
30-day supply
+ shipping & handling
3 BOXES
$44.95/ea
90-day supply
Save $15.00
6 BOXES
$39.95/ea
180-day supply
Free shipping + biggest savings
Nitric oxide supplements work best with consistent, long-term daily use. The 6-box package ensures a full 6-month evaluation period at the lowest per-box cost.
90-Day Money-Back Guarantee
CircO2 comes with a 90-day refund policy. If you don't feel a meaningful difference in your energy, stamina, or overall well-being, you can return the product — even empty boxes — for a full refund.
That's three full months to evaluate — enough time for a proper trial of any circulation support supplement. Given that NO supplements typically need several weeks of consistent use to show effects, 90 days is a reasonable evaluation window.
If You Decide to Try It
If this analysis has you considering CircO2, the 6-box package offers the best value and provides a full 6-month supply — long enough to properly evaluate the effects on your circulation and energy levels. The 3-box option is a reasonable middle ground for a 90-day trial that aligns with the guarantee period.
The 90-day guarantee removes the financial risk. If you don't notice improvement, you can return even empty boxes for a full refund.
At roughly $1.33 per day for the 6-box package, it costs less than a cup of coffee — for a doctor-formulated, dual-pathway nitric oxide supplement in a scientifically rational delivery format. The 90-day guarantee means you risk nothing by trying.
CHECK AVAILABILITY & CURRENT PRICING 🔒 Official purchase · 90-day money-back guarantee · Free shipping on 6 boxesThis link goes to the product's official page. If you make a purchase, we may receive a commission at no extra cost to you. This does not influence our editorial assessment.
Frequently Asked Questions
Nitric oxide (NO) is a signaling molecule your body produces naturally. It tells your blood vessels to relax and widen, allowing blood to flow more freely. Healthy NO levels support blood pressure, oxygen delivery to tissues, and overall cardiovascular function. Production declines significantly after age 40, which is why many people notice changes in energy and stamina as they get older.
Nitric oxide production can occur through two pathways: the L-arginine pathway (which works in the stomach and bloodstream) and the nitrate-nitrite-NO pathway (which begins in the mouth). By dissolving in the mouth, a lozenge engages oral bacteria that convert nitrates into nitrites, which then become nitric oxide. Capsules bypass this oral pathway entirely, potentially missing a significant conversion mechanism. The lozenge format is a deliberate design choice based on published NO biochemistry research.
The primary active ingredients are L-Citrulline and beetroot extract — both well-researched nitric oxide precursors. L-Citrulline supports the L-arginine/eNOS pathway, while beetroot provides dietary nitrates for the nitrate-nitrite-NO pathway. The formula also includes supporting nutrients to create a more complete environment for NO production. Each box contains 30 quick-dissolving lozenges.
Some users report feeling a difference in energy and alertness within the first week. However, cardiovascular and circulation support benefits typically require consistent daily use over several weeks to months. The 90-day money-back guarantee provides adequate time for a meaningful evaluation. We recommend committing to at least 60-90 days of daily use before drawing conclusions.
CircO2 is vegan, non-GMO, gluten-free, dairy-free, and soy-free. However, because nitric oxide directly affects blood vessel dilation and blood pressure, anyone taking blood pressure medication, blood thinners, or medications for cardiovascular conditions should consult their healthcare provider before starting this or any NO supplement. This is not optional caution — it is genuinely important.
Eating beetroot in your diet is complementary, not contradictory. CircO2 provides a standardized, concentrated dose of beetroot extract plus L-citrulline in a format designed to maximize oral pathway conversion. If you already take other NO-boosting supplements, be aware that stacking multiple products may amplify effects on blood pressure. Discuss this with your healthcare provider to ensure your total intake is appropriate for your situation.
Our Verdict
After examining the ingredients, the science behind nitric oxide supplementation, the lozenge delivery rationale, and the credibility of the formulator, CircO2 is a well-designed nitric oxide supplement with a genuinely differentiated delivery format. The dual-pathway approach — combining beetroot (nitrate pathway) with L-citrulline (L-arginine pathway) in a mouth-dissolving lozenge — is grounded in published biochemistry, not marketing claims.
The age-related decline in nitric oxide is real, measurable, and affects millions of people over 40. Whether supplementation can make a noticeable difference for you depends on your starting point — but the ingredients in this formula have more published research behind them than most supplements in this category. The lozenge format adds a layer of scientific thoughtfulness that capsule-based alternatives simply don't offer.
At $1.33/day for the 6-box package with a full 90-day money-back guarantee, CircO2 represents a reasonable, low-risk option for anyone who wants to address their body's declining nitric oxide production with a formula that has both scientific rationale and professional medical oversight behind it.