My dr just started me on 300 mg of lithium because i cannot take antidepressants and she told me i shouldn't gain weight but i think she is wrong?

Could happen: Lithium can certainly cause weight gain. But it doesn't always, and 300 mg is a small dose, making this side-effect less likely. With any medicine, you & your doctor have to weigh the risks and benefits. Most antidepressants can also cause weight gain, and several of the "atypicals" (used for bipolar, psychosis, & sometimes severe depression) can cause big weight gain. Consider your options.

Lithium & Wt. gain: Weight gain is a possible side effect of lithium and generally speaking is more likely than with common antidepressants. It is, however, less likely than with most other alternatives to treat depression when antidepressants don't work or they cannot be tolerated. Nevertheless, there is no way to predict if a person will or won't gain weight on lithium.