Very high levels of vitamin D in your blood (greater than 375 nmol/L or 150 ng/mL) can cause nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive urination and thirst, and kidney stones. Extremely high levels of vitamin D can cause kidney failure, irregular heartbeat, and even death. High levels of vitamin D are almost always caused by consuming excessive amounts of vitamin D from dietary supplements.

Excessive Dietary Intake of Vitamin A Is Associated with Reduced Bone Mineral Density and Increased Risk for Hip Fracture

Research showed multivitamin users were also more likely to have potentially excessive intakes, particularly for iron, zinc, vitamin A, and niacin.

Intakes from foods plus multivitamins were at risk of being excessive for >15% of the participants in vitamin A, niacin, and folate

The adverse effects of chronic exposure to high calcium intakes are an increase in the prevalence of kidney stones and hypercalcemia. 

Although ensuring adequacy is important, there is now an emerging issue of excess vitamin D intakes

Taking too many vitamin D supplements over a long period of time can cause too much calcium to build up in the body (hypercalcaemia). This can weaken the bones and damage the kidneys and the heart.

Pyridoxine (B6) illustrates the problems that can arise when vitamins are used at very high doses for the treatment of clinical conditions.

Research carried out in Swiss GPs showed that “Swiss GPs were unaware of many dietary supplements used by their older patients, which may affect medication optimisation efforts”, suggesting a need to understand a patients needs further.