Better Living through Chemicals? Addressing Substitution Effects in Medicine and Public Health

An interesting new study published in next month’s Journal of Health Economics examines the moral hazard involved in prevention through pharmaceutical therapy, and it has implications for the ACA’s goals of population health improvement. A convincing body of research shows that the dramatic rise in statin use over the past two decades has improved cardiovascular health by reducing the incidence of high cholesterol, serious cardiac events like AMI, and mortality from cardiovascular disease. But do statin users slack off on their nutrition and physical activity, consistent with a substitution effect, possibly leading to other health problems? Or do statin users redouble their healthy living because the drugs increase life expectancy and therefore boost the value of investing in future health status, consistent with a complementary effect?

Robert Kaestner and colleagues use longitudinal data from the famous Framingham Heart Study to investigate these questions. Using data collected on people with moderate to high cholesterol both before and after the introduction of statins in the U.S. market (1991-2001), the researchers observe changes in health behaviors associated with statin use.

Naturally, this study faces considerable risks of bias due to unobserved confounding and endogeneity because people may initiate statins due to unobserved health shocks that also influence their subsequent health behaviors. The authors use one of my favorite techniques to mitigate this bias: fixed effects estimation with a novel instrumental-variables technique. The IVs used here reflect the gradual time trend in the availability and use of statins among people who eventually become users of the drug, capturing the presumably exogenous forces that influence the diffusion pattern of a new drug. And these IVs stand up to the usual tests of strength and excludability (well, mostly…).

Results indicate that statin use led to a small BMI increase and a larger increase in the probability of being obese, consistent with the substitution hypothesis. The effect on physical activity, however, was mixed, with women reducing and men increasing their activity levels in response to statin use. Smoking was not consistently responsive to statin use, but moderate alcohol use increased among males. The authors conclude that statins serve as a strong substitute for healthy dietary intake but not for exercise or other general health behaviors that have a weaker association with cholesterol levels.

Taken as a whole, these results provide another powerful reason for combining clinical and non-clinical approaches when attempting to promote health and prevent disease on a population-wide basis. One approach used in isolation may generate behavioral offsets and unintended consequences that slow progress toward larger population health goals. As a result, medicine and public health strategies applied together may have larger effects than either strategy acting alone – the true definition of synergy. The ACA includes provisions to facilitate and incentivize these types of combined strategies, but more research is needed to determine whether these provisions work as intended. This blog will track the progress of this research, so stay tuned.

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