Side effects of intravenous lidocaine can be neurologic, cardiovascular or gastrointestinal. Toxicity can occur when used intravenously or rarely when used for anesthesia for topical purposes if a large dose is given subcutaneously.
Neurologic side effects include tremor (usually the first sign of toxicity), dizziness, dysarthria, agitation, hallucinations and drowsiness. Seizures can be caused by higher lidocaine levels since the first neurons that lidocaine suppresses are considered inhibitory neurons (inhibiting inhibitory neurons leads to neuronal overactivity and seizures).
Cardiovascular side effects include bradycardia, hypotension and asystole. These are all relatively uncommon.
Gastrointestinal side effects include nausea, vomiting, and poor appetite.