Zinc intake in excess of 300 mg per day has been reported to impair immune function.²⁸³ 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,²⁸⁴ 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.²⁸⁵

Preliminary research had suggested that people with Alzheimer’s disease should avoid zinc supplements.²⁸⁶ More recently, preliminary evidence in four patients actually showed improved mental function with zinc supplementation.²⁸⁷ 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.²⁸⁸

Zinc inhibits copper absorption. Copper deficiency can result in anaemia, lower levels of HDL (“good”) cholesterol, neurological disorders, and cardiac arrhythmias.^{289, 290, 291} Copper intake should be increased if zinc supplementation continues for more than a few days (except for people with Wilson’s disease).²⁹² 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 anaemia and suppression of bone marrow.²⁹³ 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.²⁹⁴

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 hospitalisations due to genitourinary causes (such as urinary tract infections, kidney stones, and urinary retention).²⁹⁵ 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 anaemia. In a study of women with normocytic anaemia (i.e., their red blood cells were of normal size) and low total iron-binding capacity (a blood test often used to assess the cause of anaemia), combined iron and zinc supplementation significantly improved the anaemia, whereas iron or zinc supplemented alone had only slight effects.²⁹⁶ 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.²⁹⁷