The mechanisms of Fen-Phen likely involved appetite reduction and potentially modest effects on energy expenditure, with phentermine more associated with the latter.

“I think we we suspected and still would believe that they reduced appetite and therefore helped people control their food intake better that the two things did it through slightly different uh mechanisms one being more serotonergic one being more adrenergic they may have also had effects on energy expenditure albeit probably modest ones and that was probably more the phentermine than the phenfloramine”