Im a natural and I have been running Clen for 11 days now. I ski & ski mountaineering, btw.
You might find this information useful! I like to share.
I am using sopharma brand. Its claimed to be pharma grade and the sensation from each pill is identical.
To begin with, I started with taking quarter of a pill (5ug) daily to assess response and verify the pills arent UGL junk. At this dose, I felt the immediate effects of much clearer and stronger breathing.
Resting heart rate at this dose did climb, going from 45-50 BPM to 60.
I am now running a full pill each day (20ug) and resting heart rate is 75-80.
No changes in blood pressure at all. 100/60, no changes.
No cramps, no tremors. First two days I was running HOT, the sensation of a sting around the eyes, but it fades, to the point where even at 20ug a day, I barely feel it anymore.
Each time I escalate the dose, my sleep suffers. Its a very weird stimulant. I have so much energy, but without the anxiety that comes from other stimulants. Its a "clean" energy.
To summarise, for athletic performance, I feel that the 5ug dose and the improved breathing and energy to be an effective dose for me.