Do your diet pills contain a diuretic?? Are they designed to help you get rid of water weight??
If so, they are VERY bad for you if you are on Lithium. Anything that works towards disrupting the amount of fluid you have in your body (even dehydration) can end up causing major lithium toxicity.
Mountain Dew addiction is very familiar to me!! I used to drink 8-10 cans a day. That much caffeine can be bad for the manic side of us no matter what meds we're on.
NEVER try to just quit a Mtn. Dew addiction cold turkey!! A headache from caffeine withdrawal is like having an ice pick in the middle of your forehead. Boy do I remember.
You'd need to taper down from that.
Lots of plain old ice cold water, 1 or 2 pops a day are what you're really aiming for and you'll actually feel better and you'll quench your thirst alot easier.
If you have headaches here are the ones that are NOT so good for you if you're on lithium:
ibuprofen (Advil, Motrin)
And you're not a newbie any longer, you're a Junior Member! (Been there myself a little while back!!) Welcome!