trim spa diet pills

My mom was wondering if any of you girls would happen to know why when she takes any kind of "diet" pill it has the reverse affect on her. Instead of feeling energized and "peppy" she gets really really tired. All of the diet pills out there seem to have the same effect on her. She is taking Trim Spa now (she doesn't have PCOS) and she was taking Smart Burn (which I am taking and it makes me feel great) but both make her so tired she has been taking naps during the day. I was wondering if maybe its not the diet pills but a lack of vitamins or such, but she takes a multi, fish/flax oil combo, a vitamin b complex. She is 44 if that helps any. Any kind tips or answers or anything would be great. Thanks.