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A complete guide to carboxytherapy — the technique of injecting medical-grade CO2 gas subcutaneously to trigger the Bohr effect and induce vasodilation, the evidence for under-eye dark circle improvement (hemosiderin vs. vascular vs. structural causes), stretch mark reduction, localized fat reduction, and cellulite; treatment protocol, the characteristic post-injection sensation, contraindications, and realistic expectations by indication.
· By MedSpot Editorial · 5 min read
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 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 gas is:
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 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:
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.
CO2 injection into subcutaneous fat tissue:
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.
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.
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.
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.
Absolute:
Relative:
| 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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