I had tremors, weight gain and memory problems on lithium. You need to have it about 1 - 1.2 for mood stabilisation effects. Below it only works as an antidepressant. The weight gain is often due to increased thirst ( and drinking sugary drinks) and decreased thyroid activity. So have your thyroid monitored regularly. It is normal to have blood levels checked regularly and kidney function checked often.
I'm on abilify now and love it.