Surely preventing even a handful of heart attacks,or any other negative outcome,is worthwhile——isn't it?Not necessarily.The problem is that choices that reduce some risks can put you in the path of others.Suppose the heart-attack drug caused cancer in one half of 1% of patients.In our group of 1000 people,four heart attacks would be prevented by taking the drug,but there would be five new cases of cancer.The relative reduction in heart attack risk sounds substantial and the absolute risk of cancer sounds small,but they work out to about the same number of cases.