From rxlist.

hallucinations, seizure (blackout or convulsions); fever with muscle stiffness, sweating, fast or uneven heartbeats; or. early signs of lithium toxicity, such as nausea, vomiting, diarrhea, drowsiness, muscle weakness, tremor, lack of coordination, blurred vision, or ringing in your ears. Just Google symptoms of lithium toxicity...brings up a bunch of info.