Gut Microbiome Profiles in Patients With Chemotherapy-induced Neuropathy in the RCT OzoParQT (NCT06706544).

Inclusion Criteria: * 0\. Patients who agree to participate in the randomized clinical trial OzoParQT, and who also agree to participate in this study of gut microbiota by providing stool samples. * 1\. Adults \> = 18 years old. * 2\. Previous treatment with any chemotherapy because of any tumor. * 3\. Clinical diagnosis of paresthesia (numbness, tingling) secondary to CIPN, with toxicity Grade \> = 2 (according to the Common Toxicity Criteria for Adverse Events (CTCAE) from the National Cancer Institute of EEUU, v.5.0) for \> = 3 months. * 4\. Without neurotoxic chemotherapy \> = 3 months. * 5\. Cancer disease is stable or in remission. * 6\. Life expectancy \> = 6 months. * 7\. Before enrollment, women of childbearing potential should obtain a negative result in the serum or urine pregnancy test at the screening visit and accept the use of appropriate contraceptive methods at least from 14 days before the first ozone therapy session up to 14 days after the last one. * 8\. To sign and date the specific informed consent of both studies (OzoParQT and OzoParQTmicrob) Exclusion Criteria: * 1\. Age \< 18 years. * 2\. A woman who is lactating, pregnant, suspected of being pregnant, or a woman of childbearing potential who does not use adequate contraceptive methods. * 3\. Suspected symptoms are due to diabetic or compressive neuropathy. * 4\. Severe psychiatric disorders. * 5\. Inability to complete the quality of life questionnaires. * 6\. Elevation above 5 times the maximum limit of normal creatinine. * 7\. Patient who is hemodynamic or clinically unstable or who requires urgent or short-term interventional measures. * 8\. Neoplasia in progression requiring recent initiation of systemic treatment or maintenance with neurotoxic chemotherapy. * 9\. Life expectancy (for any reason) \< 6 months. * 10\. Known allergy to ozone, known glucose 6 phosphate dehydrogenase (G6PD) deficiency, or hemochromatosis. * 11\. Contraindications or impossibility for rectal ozone treatment or to attend regularly to the treatment. * 12\. Not meeting each and every one of the inclusion criteria