Clen quickly upregulates beta-receptors, so usually it only works well for about 2 weeks.
Then you either have to cycle it or increase the dose, which also means there are increased sides, including the possibility of issues with your heart.
Clen also suppresses thyroid function, so when you come off, you will have slowed your metabolism and you will have rebound fat gain.
Far too many people take pharma fat burners without having their diet and cardio sorted, and then wonder why the drugs don't work, in fact, they may make things worse in the long run.
Personally, I think this is a comp prep drug, it is not useful at all for people who want to have a lean physique.