Systemic Review: Ozone: A Potential New Chemotherapy

Background

Conventional cancer treatments - surgery, radiotherapy and chemotherapy - remain the mainstay of care, but their efficacy varies widely between cancer types and patients, and toxicity to healthy tissue is a persistent problem. For more than six decades, researchers have periodically investigated whether medical ozone gas could offer a selective way to damage cancer cells while sparing normal ones. The proposed mechanism centres on reactive oxygen species (ROS) - highly reactive oxygen-containing molecules - which ozone is thought to generate inside cells. Cancer cells are believed to have weaker antioxidant defences than normal cells, meaning a dose of ROS that a healthy cell can neutralise may instead trigger apoptosis (programmed cell death) in a cancer cell. This review set out to gather the scattered evidence on that idea into one place and assess how far it has actually progressed.

Study design

The authors searched PubMed using the terms "ozone" and "cancer" in the title field, an initial search that returned 50 references. After screening titles and abstracts, six papers met their inclusion criteria. They then hand-searched the reference lists of those papers and of two earlier reviews on the topic, identifying a further 17 relevant studies - bringing the total to 23 publications. These comprised 11 laboratory (in vitro) studies on isolated cancer cell lines, 9 animal (in vivo) studies, and just 3 studies involving human patients. The authors describe following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines as a framework, but state openly that a full PRISMA checklist could not be completed because the included studies used such varied designs, cancer models and ozone administration methods that they could not be pooled or compared using standard systematic review methodology.

Findings

Across the in vitro studies, ozone showed a direct cytotoxic effect on several - though not all - cancer cell lines tested, including breast, lung, ovarian, colorectal, neuroblastoma and hepatocellular (liver) cancer cells, without causing comparable damage to normal cells exposed to the same doses. Where tested, ozone also appeared to have a synergistic effect when combined with established chemotherapy drugs such as 5-fluorouracil and cisplatin, and with radiotherapy. In vivo evidence was thinner: one study injecting ozonated water directly into tumours in a mouse model of rectal cancer reported tumour necrosis and inhibited proliferation without harming surrounding healthy tissue. Clinical evidence was the most limited of all - a single case series of five patients with glioblastoma (an aggressive brain cancer) treated with intratumoral oxygen-ozone reported longer survival compared with historical controls, but this was not a controlled trial and involved only a handful of patients.

What the authors concluded

The authors concluded that ozone shows a biologically plausible, ROS-mediated mechanism for selectively damaging cancer cells in laboratory settings, and that this warrants further investigation. However, they were explicit that "the translation of these results into clinical practice is far away from success." They called for substantially more preclinical work before any clinical application could be considered responsible - specifically, further animal studies using direct intratumoral delivery methods (such as arterial embolisation, intratumoral injection or catheterisation) rather than systemic administration, and dose-finding work across a wider range of cancer cell lines, since not all cancer types responded equally to ozone in the studies reviewed. Only once that groundwork is done, they argued, would randomised controlled trials in humans become appropriate.

Limitations of this study

The authors' own review has significant limitations, which they acknowledge candidly. The included studies were so heterogeneous in design, cancer model and ozone dosing that a rigorous, PRISMA-compliant systematic review and meta-analysis was not possible - this is a narrative synthesis of a PRISMA-informed search, not a pooled statistical analysis. Clinical evidence is almost nonexistent: a single five-patient case series with historical (not concurrent) controls cannot establish efficacy or safety in humans. Most of the animal studies used systemic rather than direct intratumoral ozone delivery, which the authors themselves note is a less relevant model for the intratumoral approaches they recommend going forward. Optimal dosing has not been established for any cancer type, and no funding source is disclosed for the review. This study concerns laboratory and animal research into ozone's potential mechanisms against cancer cells, together with one small, uncontrolled human case series. It does not establish ozone as a treatment or cure for cancer itself. Ozone therapy is a complementary approach and should always be discussed with your oncology team.