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You can't gain weight without eating more. According to Why Does Lithium Cause Weight Gain? lithium binds to a receptor 5-HT1B which influences the serotonine system influencing your mood, and also your appetite. This is the 1999 paper by Olivier Massot they refer to 5-HT1B Receptors: A Novel Target for Lithium: Possible Involvement in Mood Disorders.
Dry mouth is a possible side effect of lithium. If you are taking lithium and dry mouth becomes a problem, there are some things you can do. Try sucking on hard candy, sipping water or sugarless drinks often, and avoiding caffeine and alcohol.