Any ingredient in a multiple vitamin supplement can be toxic in large amounts, but the most serious risk comes from iron or calcium. Additional risks are associated with large or toxic doses of calcium, vitamin D, and vitamin A.
Vitamins A, D, E and K are fat soluble, which means they are stored in the body and if taken in high doses can be toxic. High doses of some water-soluble vitamins, such as vitamin B6, can also become toxic.
In some cases, vitamin D toxicity can result in kidney injury and even kidney failure. This is because having too much vitamin D in the body can lead to high levels of calcium, which can lead to water loss through too much urination and calcification of the kidneys ( 21 ).
You cannot overdose on vitamin B12 because any excess you simply pee out. If you get b12 shots, you may experience side effects like dizziness, nausea, and fatigue. It is possible to have elevated B12 levels in blood tests, which may indicate cancer or diabetes.
Vitamin D toxicity, also called hypervitaminosis D, is a rare but potentially serious condition that occurs when you have excessive amounts of vitamin D in your body. Vitamin D toxicity is usually caused by large doses of vitamin D supplements — not by diet or sun exposure.
Consuming vitamin B-6 through food appears to be safe, even in excessive amounts. When used as a supplement in appropriate doses, vitamin B-6 is likely safe. However, taking too much vitamin B-6 from supplements can cause: A lack of muscle control or coordination of voluntary movements (ataxia)
Although some foods contain zinc well above the UL of 40 mg daily, there are no reported cases of poisoning from naturally occurring zinc. However, zinc overdose can occur from dietary supplements or accidental excess ingestion. Zinc toxicity can have both acute and chronic effects.
Vitamin K toxicity is extremely rare. The only reported toxicity comes from menadione, which has no use in humans. Its toxicity is thought to be associated with its water-soluble properties. When toxicity does occur, it manifests with signs of jaundice, hyperbilirubinemia, hemolytic anemia, and kernicterus in infants.
Toxicity. It is unlikely to reach a toxic level of thiamin from food sources alone. In the setting of very high intakes, the body will absorb less of the nutrient and flush out any excess amount through the urine. There is no established toxic level of thiamin.
Toxicity. A toxic level of riboflavin has not been observed from food sources and supplements. The gut can only absorb a limited amount of riboflavin at one time, and an excess is quickly excreted in the urine.  Therefore, a Tolerable Upper Intake Level for riboflavin has not been established.
Common side effects include nausea, vomiting, heartburn, and esophagitis. You may also notice stomach cramps, diarrhea or intestinal obstruction. However, you can also experience symptoms such as fatigue, headaches, feeling sleepy or insomnia. Flushed skin is also a symptom.
Calcium overdose can impair the functioning of the kidneys, increase the pH of the blood, and can cause nausea and vomiting, confusion or changes in thinking or mentation, itching, and in extreme cases irregular heartbeat.
Vitamin C helps your body absorb iron and generally does not interfere or compete with other vitamins, so unlike taking calcium and magnesium — which compete with each other for absorption — you can safely combine vitamin C and zinc.
There is a lot of evidence linking zinc to blood clotting. Zinc is released from cells called platelets that control blood clotting, and scientists have found unwanted blood clots can form when zinc levels in the blood are faulty.
Acute toxicity would be caused by doses of vitamin D probably in excess of 10,000 IU/day, which result in serum 25(OH)D concentrations >150 ng/ml (>375 nmol/l). That level is clearly more than the IOM-recommended UL of 4,000 IU/day.
An excess of B6 can cause nerve damage, while excess quantities of vitamin B12 -- while rare -- may cause complications for people with certain underlying conditions. Consult your doctor before taking vitamin B6 or vitamin B12 to avoid health problems.
It is possibly safe when taken in larger doses, especially when used only for a short period of time. But taking doses higher than 40 mg daily might decrease how much copper the body absorbs. Taking very high doses of zinc is likely unsafe and might cause stomach pain, vomiting, and many other problems.