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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.
More serious side effects of lithium include blurred vision, confusion, diarrhea, drowsiness, fainting, giddiness, inability to control the bladder or bowels, increased thirst, increased or decreased urination, involuntary twitching or muscle movements, loss of consciousness, loss of coordination, muscle weakness, persistent headache, persistent or severe nausea, ringing in the ears, seizures, irregular heartbeat, slurred speech, swelling of the ankles or wrists, unsteadiness, vision changes ...