They're not the same class of drugs, so you can't compare them that way. One is a bronchiodialator, clen, and phentermine is a synthetic amphetamine derivative, like ritalin but weaker. I've used both, and for my purposes phentermine was far and away the better drug for fat loss. However, I had asthma as a child and used a LOT of albuterol inhalers, so I barely even got the shakes on 240mcg/day of clen when I weighed 180lbs. I'm basically immune to clen. Phentermine gave me energy on low-carbs, kept my cravings down too, but still let me eat enough protein and good carbs. I'd take it any day over clen, but that's just my personal view. Others may like clen/eca rotation better, I like ECY/phentermine or even ECY/ritalin if I could get it anymore. If you're going to use a drug, you may as well use one of the better ones in the class lol.