When consumed naturally through foods, these nutrients are unlikely to cause harm, even when consumed in large amounts.

Yet, when taken in concentrated doses in supplement form, it’s easy to take too much, and doing so can lead to negative health outcomes.

Side effects of overconsuming water-soluble vitamins

When taken in excess, some water-soluble vitamins can cause adverse effects, some of which can be dangerous.

However, similarly to vitamin K, certain water-soluble vitamins have no observable toxicity and hence no set UL.

These vitamins include vitamin B1 (thiamine), vitamin B2 (riboflavin), vitamin B5 (pantothenic acid), vitamin B7 (biotin), and vitamin B12 (cobalamin) (9, 10, 11, 12, 13).

It’s important to note that while these vitamins have no observable toxicity, some of them may interact with medications and interfere with blood testing results. Therefore, caution should be taken with all nutritional supplements.

The following water-soluble vitamins have set ULs, as they can cause adverse side effects when taken in high doses:

• Vitamin C. Although vitamin C has relatively low toxicity, high doses of it can cause gastrointestinal disturbances, including diarrhea, cramps, nausea, and vomiting. Migraines can occur at doses of 6 grams per day (14, 15).
• Vitamin B3 (niacin). When taken in the form of nicotinic acid, niacin can lead to high blood pressure, abdominal pain, impaired vision, and liver damage when consumed in high doses of 1-3 grams per day (16).
• Vitamin B6 (pyridoxine). Long-term overconsumption of B6 can cause severe neurological symptoms, skin lesions, sensitivity to light, nausea, and heartburn, with some of these symptoms occurring at intakes of 1-6 grams per day (17).
• Vitamin B9 (folate). Taking too much folate or folic acid in supplement form may affect mental function, negatively impact the immune system, and mask a potentially severe vitamin B12 deficiency (18).

Note that these are side effects that healthy people may experience when taking large doses of these vitamins. Individuals with health conditions can experience even more serious reactions to taking too much of a vitamin.

For example, though vitamin C is unlikely to cause toxicity in healthy people, it can lead to tissue damage and fatal heart abnormalities in those with hemochromatosis, an iron storage disorder (19).

Side effects related to overconsuming fat-soluble vitamins

Because fat-soluble vitamins can accumulate in your body’s tissues, they can cause much more harm when taken at high doses, especially over long periods.

Aside from vitamin K, which has a low potential for toxicity, the remaining three fat-soluble vitamins have a set UL due to their potential to cause harm at high doses.

Here are some side effects related to the overconsumption of fat-soluble vitamins:

• Vitamin A. While vitamin A toxicity, or hypervitaminosis A, can occur from eating vitamin-A-rich foods, it’s mostly associated with supplements. Symptoms include nausea, increased intracranial pressure, coma, and even death (20).
• Vitamin D. Toxicity from taking high doses of vitamin D supplements can lead to dangerous symptoms, including weight loss, appetite loss, and irregular heartbeat. It can also raise blood calcium levels, which can lead to organ damage (21).
• Vitamin E. High-dose vitamin E supplements may interfere with blood clotting, cause hemorrhages, and lead to hemorrhagic stroke (22).

Although vitamin K has a low potential for toxicity, it can interact with certain medications, such as warfarin and antibiotics (6).

SUMMARY

Both water- and fat-soluble vitamins can cause side effects when taken in high doses, with some causing more severe symptoms than others.