text: phentermine, which is most effective 30mg, 37.5mg or 15mg?   i think it depends on what your doctor decides is right for you. i'm on the 30. i weigh way more than my friend who goes to the same doctor as me and he's got her on the 37.5. i'm doing great on the 30 (18 lbs the first month) so i probably don't need the 37.5.  it could be a question of balancing the side effects with the weight loss too.  i'm thirsty, i get hot flashes, i can't fall asleep, can't stay asleep. my friend on the 37.5 gets worse hot flashes and even faints in the sun, and she's got constant dry mouth no matter how much she drinks. and she's losing weight slower than i am. there's a lot of info on the phentermine official site.