A 2019 review from the University of Verona gathers the laboratory, animal and human evidence that low-dose medical ozone activates Nrf2 - the protein that switches on the cell's antioxidant and protective genes.

The authors explain how Nrf2 is normally held inactive and rapidly broken down, how a mild oxidative signal releases it, and what the 200-plus genes it controls do for antioxidant defence, protein quality control, mitochondria and inflammation.

Key points: Nrf2 activation after ozone has been shown in cell studies, in animal models, and in two small human studies - healthy volunteers and people with multiple sclerosis given rectal insufflation. The authors flag an open question: over-active Nrf2 can also help tumours, and whether low-dose ozone matters here is unknown.

A 2011 review from the University of Siena describes what happens, minute by minute, when a measured dose of ozone is mixed with human blood - and why the response follows an inverted U-shaped curve.

Bocci, Zanardi and Travagli report that ozone is fully used up within about five minutes, leaving behind hydrogen peroxide and lipid oxidation products that act as messengers. The blood's antioxidant capacity dips and then recovers within about 20 minutes.

Key points: too small a dose is neutralised with no effect, a dose inside the therapeutic window produces the benefit, and too large a dose brings side effects such as fatigue. The authors recommend a "start low, go slow" approach and caution that the curve has only been shown for peripheral arterial disease.

A 2011 review by Masaru Sagai and Velio Bocci sets out why ozone damages the lungs yet can be used therapeutically in blood - and proposes that its benefits come from a brief, mild oxidative stress that switches on the body's own defences.

The authors describe how ozone reacts within moments with antioxidants and fats in body fluids, producing two short-lived messengers - hydrogen peroxide and lipid oxidation products - that carry the signal onwards.

Key points: a moderate oxidative signal is proposed to activate Nrf2, the switch for the cell's antioxidant enzymes, while a severe one drives inflammation through NF-kB. The authors compare the effect to exercise and stress that ozone is not a cure-all and must never be inhaled. Narrative review; the Nrf2 link was a hypothesis at the time.

A randomised, double-blind, placebo-controlled trial of 30 women with endometriosis tested weekly endovaginal medical ozone for 10 weeks as an adjuvant to their existing care.

Pain, endometriosis-specific quality of life, sexual function and anxiety were measured with validated questionnaires before and after the course. The trial's registry entry specifies 1,000 mL of ozone-oxygen gas at 30 µg/mL per session, with a sham probe as placebo.

Key findings: the ozone group showed significant reductions in pain and significant improvements in quality of life, sexual function and anxiety compared with placebo, with no serious adverse events. Small single-centre study; full results are paywalled and effect sizes are not reproduced here.

A controlled trial on tomato seedlings compared ozonated water applied two ways - watered into the soil, and sprayed onto the leaves - across a range of concentrations.

Seedlings of the cultivar 'Little Tom' were irrigated with ozonated water at 1.0 to 4.0 mg/L, or sprayed at 0.2 to 0.8 mg/L, and compared against plain water. The researchers measured cell-membrane damage, antioxidant enzyme activity, leaf greenness and nitrogen content, then examined gene expression after the best-performing concentrations.

Key findings: Irrigation at 3.0 mg/L performed best overall, reducing the cell-damage marker malondialdehyde by 53% and raising antioxidant enzyme activity substantially. Spraying worked too, at a much lower optimum of 0.6 mg/L, but the authors found irrigation the more effective route. Irrigation at 4.0 mg/L performed worse than 3.0.

Note: A small study - five plants and three biological replicates per treatment - on one tomato cultivar in one soil type.

A randomised study of 48 adults with constipation-predominant irritable bowel syndrome tested ozonated water and rectal insufflation added to standard treatment.

All patients received drug therapy and interference therapy; 28 also received ozonated distilled water three times daily for three weeks plus eight rectal insufflations. Colonic microflora, barium transit time and SF-36 quality of life were measured before and after.

Key findings: Bifidobacteria and lactobacilli improved in 73.7% of the ozone group against around 47% of controls, with better results for Candida, Staphylococcus aureus and haemolysing E. coli. Radiological improvement was seen in 89.5% of the ozone group against 60.5% of controls, and quality of life scores were significantly higher.

A preclinical rat study examined how ozone therapy affects immune markers at the site of intestinal injury in experimental ulcerative colitis.

Sixty-four Wistar rats with oxazolone-induced colitis received an ozone-oxygen mixture either intraperitoneally or rectally over six days, with IL-17, IL-23, CD4-positive lymphocytes and FoxP3-positive regulatory T cells measured in intestinal tissue.

Key findings: Rectal ozone significantly reduced both IL-17 and IL-23 by day 6 compared with untreated animals, and returned FoxP3-positive cell numbers to healthy levels. Intraperitoneal ozone normalised lymphocyte populations but reduced only IL-23.

Note: This is an animal study. It is not evidence of effect in humans.

A randomised controlled trial compared rectal ozone insufflation against standard treatment in 40 patients with IgA deficiency.

IgA deficiency is a primary immunodeficiency with no replacement therapy available. This phase 2 trial, conducted at a Cuban provincial hospital, measured immunoglobulin levels, pro-oxidant and antioxidant biomarkers, and clinical response before and after two cycles of rectal ozone.

Key findings: Significant increases in IgG and IgM in the ozone group, alongside reduced oxidative stress markers and increased antioxidant enzymes - none of which occurred in the control group. Complete therapeutic response was recorded in 85% of the ozone group compared with 45% of controls.

Ozone treatment improved quality of life and reduced treatment toxicity in cancer survivors living with persistent side effects.

This preliminary study assessed 26 cancer survivors with chronic, refractory side effects of radiotherapy and chemotherapy, measuring health-related quality of life and graded toxicity before and after ozone treatment.

Key findings: Significant improvement across all five quality-of-life dimensions - mobility, self-care, usual activities, pain and discomfort, and anxiety and depression - alongside a significant reduction in the grade of treatment-related toxicity.