The action was based on findings from doctors who had evaluated people taking these two drugs with echocardiograms, a procedure that can test the functioning of heart valves. The findings indicated that approximately 30 percent of people who had taken the combination up to 24 months had abnormal echocardiograms, even though they had no symptoms. This percentage of abnormal test results was much higher than would be expected from a comparatively sized sample of the population who had not been exposed to either fenfluramine or dexfenfluramine. Follow up studies showed that for people who took the combination for 3 months or less, the rate of heart valve complications was less than 3%.