Vitamin B12 has long been seen as a health hero, helping the body make red blood cells, repair DNA, and keep nerves functioning properly. But scientists are discovering that the story may be more complicated than simply โ€œmore is better.โ€ While too little B12 can damage DNA and raise cancer risk, some studies suggest that extremely high levels โ€” especially from long-term high-dose supplements โ€” may also be linked to certain cancers or poorer outcomes in cancer patients.