This is an example of healthy user/healthy adherer effect. This effect is very difficult to adjust for, and its impact pervades the nutrition epidemiological literature.
In this study of 141,086 patients initiating statins for primary prevention in Canada, it was hypothesized that patients who were more adherent would be at decreased risk of accidental health events.
Statins do not reduce the risk of motor vehicle accidents. What’s more plausible is that the people who take statins are less likely to be in motor vehicle accidents.
The healthy-user effect is a hypothetical source of confounding bias that is thought to affect observational studies of drugs, diets, screening procedures, and other health-related behaviors. This bias presumes that patients who initiate and adhere to preventive therapies are more likely to engage in behaviors consistent with a healthy lifestyle than are patients who do not initiate or adhere to such treatments. Aspects of a healthy lifestyle could include diet, exercise, moderation of alcohol, and avoidance of risky behaviors.Dormuth CR, Patrick AR, Shrank WH, Wright JM, Glynn RJ, Sutherland J, et al. Statin Adherence and Risk of Accidents. Circulation [Internet]. 2009 Apr 21;119(15):2051–7. Available from: https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.108.824151
Reference
Dormuth CR, Patrick AR, Shrank WH, Wright JM, Glynn RJ, Sutherland J, et al. Statin Adherence and Risk of Accidents. Circulation [Internet]. 2009 Apr 21;119(15):2051–7. Available from: https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.108.824151