Phentermine and Ionamin are the same drug-the difference is that Ionamin is the time released form (resin based), phen is not. I've never taken Ionamin, so I can't compare the two personally. Years ago, my sister and I wanted to start a plan to lose weight together-I did phen, and my sister did Ionamin. I lost 15 pounds the first month, she didn't lose any. I ended up losing 45-50 pounds in about 3 months I think. She didn't (and still doesn't) want to do phentermine, because the side effects may be stronger without the time release.....you kind of "get it all at once" with phen, and that causes some people to get jittery. I never had a problem with it, but you can get a lower dose (I took the strongest, 37.5 mg) or split the pills and take 1/2 in the morning and afternoon if that bothers you.