Healthy-User Bias: Why the People Who Take the Supplement Were Already Going to Live Longer

Summary

The people who take the vitamin, stick to the regimen, get the screening, or show up to exercise are systematically different from those who don't — wealthier, more health-conscious, less sick to begin with — so "users of X live longer" observational findings often measure *who chooses X*, not *what X does*; the cleanest proof is that adherence to a **sugar pill** predicted survival almost as strongly as adherence to the real drug.