As with any sort of stimulant, clen can damage the heart at high doses. However, if the heart muslce is strong, the damage won't be so severe, but someone would have to be a absolute moron to do anything else, but start at the absolute lowest dosage of clen, or any other stimulate till you see how your body reacts. A lot of people's hearts are not used to the constant faster hard beating that clen provides, that's why its best to start out low. However, there are a lot of mornons out there that do not do there research and start out to high, and that is when people have problems.
If someone does have a heart problem, than yes any sort of stimulant in the corect dosage can induce a myocardial infraction. However, with clen, any dose can be to high depending on the person, you have to test your tolerance. .25mcg of clen works awesome for me, if I went 50mcg, I might be a bit concerned with heart damage, that's why its best to test the waters slowly with this stuff.