Zinc intake in excess of 300 mg per day has been reported to impair immune function.7 Some people report that zinc lozenges lead to stomach ache, nausea, mouth irritation, and a bad taste. One source reports that gastrointestinal upset, metallic taste in the mouth, blood in the urine, and lethargy can occur from chronic oral zinc supplementation over 150 mg per day,8 but those claims are unsubstantiated. In topical form, zinc has no known side effects when used as recommended.
Caution: Using zinc nasal spray has been reported to cause severe or complete loss of smell function. In some of those cases, the loss of smell was long-lasting or permanent.9
Preliminary research had suggested that people with Alzheimer's disease should avoid zinc supplements.10 More recently, preliminary evidence in four patients actually showed improved mental function with zinc supplementation.11 In a convincing review of zinc/Alzheimer's disease research, perhaps the most respected zinc researcher in the world concluded that zinc does not cause or exacerbate Alzheimer's disease symptoms.12
Zinc inhibits copper absorption. Copper deficiency can result in anemia, lower levels of HDL ("good") cholesterol, neurological disorders, and cardiac arrhythmias.13, 14, 15 Copper intake should be increased if zinc supplementation continues for more than a few days (except for people with Wilson's disease).16 Some sources recommend a 10:1 ratio of zinc to copper. Evidence suggests that no more that 2 mg of copper per day is needed to prevent zinc-induced copper deficiency. Many zinc supplements include copper in the formulation to prevent zinc-induced copper deficiency. Zinc-induced copper deficiency has been reported to cause reversible anemia and suppression of bone marrow.17 In addition, there are case reports of neurologic abnormalities due to copper deficiency occurring in people who had been using large amounts of certain widely available denture creams that contained high concentrations of zinc.18
In a study of elderly people with macular degeneration, supplementing with 80 mg of zinc per day for an average of about six years increased by about 50% the incidence of hospitalizations due to genitourinary causes (such as urinary tract infections, kidney stones, and urinary retention).19 In that study, copper was also given, but in a form that cannot be absorbed by humans (cupric oxide). The reported adverse effect of zinc may have been due in large part to zinc-induced copper deficiency, which could be prevented by taking copper in a form other than cupric oxide. Nevertheless, it would be prudent for elderly people wishing to take large amounts of zinc to consult with a doctor.
Marginal zinc deficiency may be a contributing factor in some cases of anemia. In a study of women with normocytic anemia (in other words, their red blood cells were of normal size) and low total iron-binding capacity (a blood test often used to assess the cause of anemia), combined iron and zinc supplementation significantly improved the anemia, whereas iron or zinc supplemented alone had only slight effects.20 Supplementation with zinc, or zinc and iron together, has been found to improve vitamin A status among children at high risk for deficiency of the three nutrients.21