I have made 2-3 attempts with clen, and never make it beyond 3-4 days. I have dosed it quite conservatively each time, but still end up lightheaded, SOB, or feel like my heart is pounding out of my chest. It just feels too dangerous for me.
Part of the problem is that it also makes me tired, so I maintain my use of stimulants. My body lets me know that it is not impressed with that scenario.