Reference summary — not medical advice or a personal risk estimate.
Helicobacter pylori
Quick answer
Classified as Group 1 — carcinogenic to humans, based on sufficient evidence in humans. Infection. Lives in the stomach lining of about half the global population; passes between people, often in childhood. Testable and treatable with antibiotics.
Summary
A stomach bacterium that infects about half of the world's population. Associated with stomach cancer and a stomach lymphoma. Treatable with antibiotics.
Detail
H. pylori is Group 1 with sufficient evidence for non-cardia gastric cancer and gastric MALT lymphoma. WHO and IARC estimate roughly half of the global population is colonised; prevalence is highest in low- and middle-income regions. Infection can be detected by breath, blood or stool test, and is typically eradicated with a course of antibiotics plus acid-suppressing medication.
What's not established
This classification is based on sufficient evidence in humans that this factor can cause cancer. It does not establish a safe exposure threshold, your personal risk, or how this factor's risk compares in size to other Group 1 agents — hazard classifications are not ranked by magnitude.
Regional relevance
global
Evidence sources
Every claim on this page traces back to one of these. Click through to read the primary document yourself.
Primary sources
NCI
NCI — Risk Factors: Infectious Agents
IARC
IARC Monographs — Classifications by Cancer Site, Vols 1–140
Supporting sources
ECDC
ECDC — Communicable diseases and cancer risk (European Centre for Disease Prevention and Control)
Canadian Cancer Society
Canadian Cancer Society — Reduce your risk
ACS
ACS — Known and Probable Human Carcinogens
Cancer Research UK
Cancer Research UK — Causes of cancer and reducing your risk
IARC / WHO — European Code Against Cancer
European Code Against Cancer (5th edition) — 14 ways you can help prevent cancer
NTP
NTP — Report on Carcinogens (US National Toxicology Program)
Cancer Council Australia
Cancer Council Australia — Causes of cancer
Last reviewed: October 2026
Official reference list
A government list some readers use alongside the primary evidence. Meyron is not affiliated with it. A chemical being listed does not by itself describe a risk from any particular product.
- California Prop 65 list
State of California's list of chemicals known to cause cancer or reproductive harm.