Research suggests that people with blood group O may have a slightly lower risk of developing certain cancers than those with blood groups A, B, or AB. A large review summarized the evidence by stating, **“Blood group A is associated with increased risk of cancer, and blood group O is associated with decreased risk.”**

The strongest link appears with stomach and pancreatic cancers. Studies indicate that people with blood group A or AB are more likely to develop gastric cancer than those with blood group O, while pancreatic cancer is also reported more often in non-O blood groups. One analysis found that people with blood group A had about a **19 percent higher risk of stomach cancer** compared with group O.

Scientists also note that results can differ between populations. For example, one study found that Chinese men with blood group B had a lower risk of gastrointestinal cancers than those with blood group A, suggesting that genetics and regional differences may influence the findings. At the same time, research on cancers such as breast cancer has found little or no meaningful connection with ABO blood type.

Although blood group O appears to have the lowest average cancer risk, the differences are relatively small and should not be seen as protection from disease. As researchers emphasize, **“these are associations, not proof of causation.”** Many other factors, including diet, smoking, infections such as *H. pylori*, family history, and environmental exposures, have a much greater impact on cancer development.

Overall, current evidence suggests that blood group O is linked with a modestly lower risk for some cancers, especially stomach and pancreatic cancer. However, **“Having blood group O does not guarantee protection”**, and healthy lifestyle choices, regular medical checkups, and recommended cancer screenings remain far more important for reducing overall cancer risk.