For me the Phen worked the very first day. It worked better for the late night munchies when I took it before lunch instead of before breakfast. Today I decided to give the Didrex one more try so I took 1 at 8am 1 at 12noon and one at 3pm. Didn't feel a thing and I've been hungry all day. And I mean stomach growling, not just bored and want to eat. In fact I worked all day today and on the Phen I wouldn't even think about food. My personal opinion is that didrex is crap.
I've read somewhere that 5htp extends the life of Phen.