In one study, administration of 1.9 g per day of calcium for 6 weeks to healthy individuals reduced the uptake of iodine by the thyroid gland. Further studies suggested that this effect of calcium was due to inhibition of gastrointestinal absorption of iodine. To be on the safe side, some doctors recommend that people taking calcium for long periods of time should also take a multimineral supplement that contains iodine.

Supplementing with folic acid in amounts as little as 300 to 800 mcg per day may increase vitamin B12 requirements. Therefore, it would be prudent for people taking folic acid to supplement with vitamin B12.

Supplementation with vitamin B12, even in relatively small amounts, can increase folic acid requirements, For that reason, people taking vitamin B12 supplements should also consider taking folic acid.

In addition, supplementing with folic acid can mask the laboratory diagnosis of vitamin B12 deficiency. Failure to identify and treat vitamin B12 deficiency in a timely manner can lead to permanent neurological damage. The relatively small amount of vitamin B12 present in most supplements is not sufficient to prevent or correct vitamin B12 deficiency in people who have pernicious anemia (a condition caused by vitamin B12 malabsorption). For that reason, people should tell their doctor if they are taking folic acid supplements, so that the doctor will order alternative laboratory tests if vitamin B12 deficiency is suspected.

High phosphorus intake may impair absorption of iron, copper, and zinc.

Vitamin C appears to convert sodium selenite (a form of selenium used for supplementation) to elemental selenium, making it unavailable for absorption. This interaction occurred when 1 g of vitamin C was taken with sodium selenite on an empty stomach, but not when these nutrients were taken together with a meal. Vitamin C does not appear to interact with selenomethionine, another commonly used form of supplemental selenium.

Zinc competes for absorption with copper  and inhibits copper absorption. Copper deficiency can result in anemia, lower levels of HDL (“good”) cholesterol, neurological disorders, and cardiac arrhythmias. Copper intake should be increased if zinc supplementation continues for more than a few days (except for people with Wilson’s disease). 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. 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.

In a study of healthy men, supplementation with a large amount of zinc (142 mg per day) decreased magnesium absorption. Magnesium supplementation should therefore be considered for people taking large amounts of zinc, and possibly for those taking lower amounts.