Aversion to side effects in preventive medical treatment decisions

OBJECTIVES: Individuals may be overly sensitive to the side effects of treatments aimed at preventing illness, in part because they have difficulty in evaluating situations with several possible outcomes that differ in probability. This study tested willingness to undergo a hypothetical preventive treatment and accuracy in determining the probability of harm from the treatment as a function of the presence of a side effect, the initial probability of harm, the format in which probabilities were presented (percentages or frequencies), and the presence or absence of a graphic. DESIGN: The study was a factorial experiment involving 5,251 participants. METHODS: Participants recruited from a health-oriented internet site read about a hypothetical cancer prevention treatment situation and were asked to indicate their willingness to accept this treatment and whether it would increase or decrease their overall risk of cancer. The net benefit of the treatment was the same in all conditions, whether or not it was associated with a small side effect. RESULTS: The presence of information about a side effect dramatically decreased willingness to undergo preventive treatment and accuracy in evaluating the treatment's effects. Willingness and accuracy were not influenced by the initial probability of harm, whether the risk probability information was presented as frequencies (N in 100) or as percentages, or whether the initial risk was presented with a bar graph or an array of asterisks or stick figures. CONCLUSIONS: Individuals are highly averse to preventive treatments with even small side effects and have difficulty combining the likelihood of positive and negative outcomes to determine the treatment's overall benefits.