Just had to bump this thread and say that everyone who spoke out against clen use for endurance is correct. Maybe off season like after you had way to much turkey and your gonna lay around but even then I don't like the fact that prolonged use can build up hard tissue around the heart. I'm a cyclist and tried clen (very small doses) for two days and hated it. It made my heart rate rise, where as before it was low and strong. Made me feel short of breath (I know this is supposed to be for asthma, go figure) and that is something you do not want when competing in endurance sports.