6 Side Effects of Too Much Vitamin D
1. Elevated Blood Levels
Achieving adequate levels of vitamin D in your blood may help boost immunity and protect you from diseases like osteoporosis and cancer (5).
However, there isn't universal agreement on the optimal range for these levels.
Although a vitamin D level of 30 ng/ml (75 nmol/l) is typically considered adequate, the Vitamin D Council recommends maintaining levels of 40–80 ng/ml (100–200 nmol/l), and states that anything over 100 ng/ml (250 nmol/l) may be harmful (6, 7).
Despite the fact that more people are now taking vitamin D supplements, it's rare to find someone with very high blood levels of this vitamin.
One recent study looked at data from more than 20,000 people over a 10-year period. It found that only 37 people had levels above 100 ng/ml (250 nmol/l). Only one person had true toxicity, at 364 ng/ml (899 nmol/l) (8).
2. Elevated Blood Calcium Levels
Vitamin D helps your body absorb calcium from the food you eat. In fact, this is one of its most important roles.
However, if vitamin D intake is excessive, blood calcium may reach levels that cause symptoms that are not only unpleasant, but dangerous.
Symptoms of hypercalcemia, or high blood calcium levels, include:
Digestive distress, such as vomiting, nausea and stomach pain
Fatigue, dizziness and confusion
The normal range of blood calcium is 8.5–10.2 mg/dl (2.1–2.5 mmol/l).
In one case study, an older man with dementia who received 50,000 IU of vitamin D daily for six months was repeatedly hospitalized with symptoms related to high calcium levels (10).
In another, two men took improperly labeled vitamin D supplements, leading to blood calcium levels of 13.2–15 mg/dl (3.3–3.7 mmol/l). What's more, it took a year for their levels to normalize after they stopped taking the supplements (11).
3. Nausea, Vomiting and Poor Appetite
Many side effects of too much vitamin D are related to excessive calcium in the blood.
These include nausea, vomiting and poor appetite.
However, these symptoms don't occur in everyone with elevated calcium levels.
One study followed 10 people who had developed excessive calcium levels after they had taken high-dose vitamin D to correct deficiency.
Four experienced nausea and vomiting and three had a loss of appetite (12).
4. Stomach Pain, Constipation or Diarrhea
Stomach pain, constipation and diarrhea are common digestive complaints that are often related to food intolerances or irritable bowel syndrome.
However, they can also be a sign of elevated calcium levels caused by vitamin D intoxication (15).
These symptoms may occur in those receiving high doses of vitamin D to correct deficiency. As with other symptoms, response appears to be individualized even when vitamin D blood levels are similarly elevated.
In one case study, a boy developed stomach pain and constipation after taking improperly labeled vitamin D supplements, whereas his brother experienced elevated blood levels without any other symptoms (16).
5. Bone Loss
Because vitamin D plays an important role in calcium absorption and bone metabolism, getting enough is crucial for maintaining strong bones.
However, too much vitamin D can also be detrimental to bone health.
Although many symptoms of excessive vitamin D are attributed to high blood calcium levels, some researchers suggest that megadoses may lead to low levels of vitamin K2 in the blood (18).
One of vitamin K2's most important functions is to keep calcium in the bones and out of the blood. It's believed that very high vitamin D levels may reduce vitamin K2 activity (18, 19).
6. Kidney Failure
Excessive vitamin D intake frequently results in kidney injury.
In one case study, a man was hospitalized for kidney failure, elevated blood calcium levels and other symptoms that occurred after he received vitamin D injections prescribed by his doctor (20).
Indeed, most studies have reported moderate-to-severe kidney injury in people who develop vitamin D toxicity (9, 12, 13, 14, 16, 17, 21).
In one study of 62 people who received excessively high-dose vitamin D injections, each person experienced kidney failure — whether they had healthy kidneys or existing kidney disease (21).
The Bottom Line
Vitamin D is extremely important for overall health. Even if you follow a healthy diet, you may require supplements in order to achieve optimal blood levels.
However, it is also possible to have too much of a good thing.
Make sure to avoid excessive doses of vitamin D. Generally speaking, 4,000 IU or less per day is considered safe as long as your blood values are being monitored.
In addition, make sure you purchase supplements from reputable manufacturers to reduce the risk of accidental overdose due to improper labeling.
If you've been taking vitamin D supplements and are experiencing any of the symptoms listed in this article, consult a healthcare professional as soon as possible.