Signs of toxicity are nausea, vomiting, diarrhea, fever and lethargy and have been observed after ingestion of 4-8 g zinc according to a 2002 WHO report. Single doses of 225-450 mg of zinc usually induce vomiting.
Doses of zinc ranging from 100 to 150 mg/day interfere with copper metabolism and cause hypocuprinaemia, red blood cell microcytosis and neutropenia if used long term.
Mild gastrointestinal distress has been reported at doses of 50-1 50 mg/day of supplemental zinc. According to a randomised, double-blind study, zinc gluconate glycine lozenge (104 mg equivalent to 13.3 mg ionic zinc) taken every 3-4 hours is well tolerated. Of the side-effects that were reported, dry mouth and a burning sensation on the tongue were probably related to use, whereas symptoms of nausea, dizziness, lightheadedness and upset stomach were considered as possibly related.