Carboxytherapy guide: CO2 injections for dark circles, stretch marks, and body contouring

Carboxytherapy is the therapeutic injection of medical-grade carbon dioxide (CO2) gas into subcutaneous tissue or intradermally, producing local physiological effects through the Bohr effect and vasodilation. It has the broadest evidence base among gas-injection aesthetic treatments and a well-understood mechanism. Here is a complete evidence-based guide by indication.

The mechanism: the Bohr effect

Why CO2 injection works

The central mechanism of carboxytherapy is the Bohr effect — a fundamental principle of oxygen-hemoglobin physiology:

When CO2 concentration increases in tissue, hemoglobin's affinity for oxygen decreases (the oxygen-hemoglobin dissociation curve shifts rightward). Hemoglobin releases oxygen more readily in high-CO2 environments.

The cascade following CO2 injection:
CO2 is injected subcutaneously → local CO2 concentration rises dramatically
Bohr effect activates → hemoglobin releases oxygen more readily in the injection zone
Local tissue oxygen tension rises → improved aerobic cellular metabolism
CO2 acts as a direct vasodilator → arteriolar vasodilation → increased local blood flow
Increased oxygen delivery + increased perfusion → fibroblast activation, collagen synthesis, and improved tissue metabolism

CO2 gas is:
Naturally present in all tissues (end-product of aerobic metabolism)
Completely non-toxic and rapidly absorbed — diffuses into the bloodstream and is exhaled within minutes
Not air (which contains nitrogen and can cause gas embolism) — medical-grade CO2 is safe for subcutaneous injection in small volumes via controlled carboxytherapy equipment

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Indications and evidence

Under-eye dark circles

Under-eye dark circles have three distinct etiologies — carboxytherapy addresses two of them:

Vascular dark circles (most common): The periorbital skin is the thinnest on the face (0.5 mm). The dense vascular network beneath shows through as bluish or purplish discoloration. CO2 injection around the periorbital area induces vasodilation → increased blood flow → improved oxygenation of sluggish, deoxygenated blood pooled in the vascular network → reduction in visible blue/purple discoloration.

Hemosiderin / post-inflammatory dark circles: Iron deposits from microbleeds in the periorbital tissue appear as brownish discoloration. The improved tissue metabolism and fibroblast activity from CO2 injection supports gradual clearance of hemosiderin deposits over a series.

Structural/hollowing (tear trough) dark circles: CO2 does not address volume loss or tear trough hollowing — this requires HA filler or Sculptra.

Evidence: Mehryan et al. (2014, Journal of Cosmetic Dermatology) — randomized controlled split-face study of 28 patients with dark circles; CO2 injection significantly improved dark circle severity scores on the treated side vs. control. Ferreira et al. (2019, Dermatologic Surgery) — photographic improvement documented in periorbital hyperpigmentation and vascular pooling after carboxytherapy series.

Stretch marks (striae distensae)

Stretch marks represent areas of dermal scarring with disrupted collagen architecture, thin epidermis, and poor vascularity. The classic pale, depressed striae alba have poor response to most topical treatments because of this avascular, fibrotic state.

CO2 injection into stretch marks:
Dramatically increases local blood flow to the otherwise poorly perfused scar tissue
Activates fibroblasts in the peri-striae dermis → new collagen deposition
Mechanical distension from gas injection creates micro-trauma → wound-healing response

Evidence: Piccinato et al. (2021, Journal of Cosmetic and Laser Therapy) — CO2 injection into striae distensae produced significant improvement in width, depth, and coloration vs. control in split-body RCT. Multiple Brazilian dermatology groups (where carboxytherapy is extensively practiced) have published case series showing consistent improvement, particularly in striae alba.

Realistic expectation: Carboxytherapy improves stretch marks — reduces width, increases collagen density, improves color — but does not fully eliminate established striae. A series of 6–10 sessions produces the best results; combination with fractional laser or microneedling augments outcome.

Localized fat reduction and cellulite

CO2 injection into subcutaneous fat tissue:
Induces lipolysis through increased adipocyte metabolism (the higher oxygen delivery and perfusion activates metabolic activity)
May trigger apoptosis of adipocytes at high local CO2 concentrations
Improves microcirculation in cellulite-affected tissue → reduces the fibroseptal tethering appearance

Evidence (body): Several Brazilian studies document circumference reduction after carboxytherapy series for localized adiposity; effect size is modest (1–3 cm circumference reduction) and less than cryolipolysis or HIFU for significant fat reduction.

Cellulite: Improved microcirculation and partial reduction in fibroseptal tethering produce visible but temporary improvement in cellulite grade. Carboxytherapy is more effective for cellulite than most non-invasive interventions, but results require maintenance.

Alopecia (scalp carboxytherapy)

Scalp CO2 injection improves microcirculation and oxygen delivery to hair follicles — used as an adjunct for androgenetic alopecia and telogen effluvium. The evidence is moderate; most robust as a combination treatment with minoxidil or PRP rather than monotherapy.

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Treatment experience

The injection sensation

Carboxytherapy has a characteristic and notable treatment experience:

The inflation sensation: As CO2 gas is injected, a rapidly expanding pressure/fullness is felt at the injection site — distinct from liquid injection. This resolves as the gas diffuses (typically within minutes).

The effervescence: Many patients describe a mild tingling or "carbonated" sensation at the injection site as CO2 diffuses through tissue.

Post-injection erythema: The vasodilation from CO2 produces visible flushing/erythema at the treatment area — particularly dramatic under the eyes. This resolves within 30–60 minutes and is a sign the mechanism is working.

Discomfort: 3–5/10 without topical anesthetic; primarily from the pressure sensation of gas injection. Under-eye injections are more sensitive. Topical EMLA reduces discomfort.

Protocol by indication

Under-eye dark circles: 2–3 mL CO2 per side per session; 6–8 sessions at weekly intervals; results begin at 3–4 sessions.

Stretch marks: Injections along each striae at 1–2 cm intervals; 6–10 sessions at weekly intervals.

Body fat / cellulite: Higher volumes (50–100 mL per area per session); 8–12 sessions at 1–2 week intervals.

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Contraindications

Absolute:
Respiratory compromise (severe COPD, emphysema): CO2 loads must be exhaled — compromised respiratory function cannot clear the additional CO2. Do not use in patients with severely reduced respiratory reserve.
Active skin infection at injection sites
Known hypersensitivity to CO2 (extremely rare; essentially theoretical)

Relative:
Pregnancy: Insufficient safety data
Sickle cell disease: Bohr effect shift in already-compromised red cells
Cardiac arrhythmias: Large volumes of CO2; caution
Active Raynaud's phenomenon: Though carboxytherapy is sometimes used to treat Raynaud's, acute episodes are a contraindication for treatment sessions

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Realistic expectations by indication

| Indication | Evidence Level | Effect Size | Sessions Needed | Durability |
|---|---|---|---|---|
| Vascular dark circles | Moderate–strong | Moderate | 6–8 | 6–12 months with maintenance |
| Hemosiderin dark circles | Moderate | Moderate | 8–10 | Variable |
| Stretch marks | Moderate | Moderate | 6–10 | Long-lasting (collagen) |
| Localized fat | Moderate | Modest | 8–12 | Requires maintenance |
| Cellulite | Moderate | Moderate | 8–12 | 3–6 months |

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