The racing heart used to happen to me, especially when I first started. It doesn't happen now for some reason. It only freaked me out once, but I realized that it was mostly psychological. Yes, your heart rate does slightly raise, but it's not enough to cause your heart to fail... definitely not. Just don't think about it (I know, it's harder than I make it sound). Don't worry about it, unless you're mixing it with another medication. That, on the other hand, can possibly cause problems. I know some people can have trouble with smoking weed and taking Effexor.