I definitely take a cardio hit when I'm on. Didn't always be like that when I was younger. but I've been off for 3 weeks and my stamina is right back up to par. Personally I think it's transient and if you wither go off or get off everything except for a normal replacement for periods of a few months on and off you should be fine. Do big doses for long long periods and there can be problems. BP goes up, heart rate goes up. blood gets thick. Vascular endothelium gets thick and stiff. Not good things. You want to be national caliber if a pro then there is going to be some dysfunction. Maybe reversible or maybe not. Today you really have to push the envelope. But as a gym guy or a lower level BBer there is really no need to be pounding away on the syringe lever non-stop year after year. Take breaks, use your head, listen to your body and you should be fine.