It depends on the type of slimming 'aid' you are planning taking...
Something like Reductil / Accomplia works on your brain in a very similar way to Prozac - In the patient advice leaflet from Reductil it says to have a break of at least two weeks between each different type of drug.
Probably to give your body time to recover!
Its all to do with the chemical signals in your brain - and they borth kinda work on the same frequency so it could get a bit messy... I've never taken the two together but often have taken either one or the other.
Xenecal has no effect on these tablets, but i was told by the specialist at the hospital that Fluoxetine can have a mildy irritant effect on your bowels. Not the best of problems to have when taking Xenecal as it works primarily on it and the fat absorption within.
Personally, I wouldnt risk it if thats the kind of thing you are thinking of taking.
Any pharamcist can give you advice on what drugs do and don't go together - even if they have to go and look it up!
Hope it helps
:-)