The suggested safe level of supplementation is 0.4 mg per day. More than 5 mg per day may mask anaemia caused by Vitamin B12 deficiency, and interfere with zinc bioavailability. Thus it is often wise to accompany folate supplementation with B12 supplementation. But an excess of Vitamin C can cause diarrhoea, kidney stones, reflux, dental erosions and gastrointestinal obstruction. Vitamin D in excess can cause hypercalcaemia and birth defects.
Excessive intake can lead to iron overload in susceptible subjects.
Iron may be deposited in the heart, pancreas and liver, leading to disease in these organs. Iron excess can also interfere with zinc absorption. In excess, zinc may decrease immunity and inhibit the absorption of copper, an essential mineral. But an excess can turn it into a harmful pro-oxidant, which decreases immune function. On the other hand, more than 600 micrograms per day may cause nail, skin, teeth and nerve toxicity.