Yes It has to do with circulatory system, but heart attack is alittle extreme, but there can be VERY RARE occasioins .I would stop taking the pills lay off caffine for 2-3 days and take ZMA before bed. When ever people start fat burners they tend to depelete minerals (potassium, zinc, magnesium, b complexes) from there body faster then they can be replaced at the same time stimulating the central nervous system to go into stress mode. Given the fact that athletes despite all the vitamins and nutrients we take we are still lagging. Add on top the fat burners, daily stress, dieting, and your body will use up nutrients faster then a hooker can make you cum.