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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.
High lithium levels can cause shakiness, muscle weakness, slurred speech, nausea or vomiting, and drowsiness.  Keep your caffeine intake about the same. Keep amounts of coffee, tea, cola, and other soft drinks with caffeine about the same from day to day. Less caffeine can cause your lithium level to increase; more caffeine can cause your lithium level to decrease. Avoid caffeine because the effect of these drugs may be changed if taken within 40 minutes of caffeine.