Vitamin Mega-Doses

Can you get too much of a good thing?

Every year Americans spend billions of dollars on vitamins, minerals and herbal supplements. Usually, the amounts we take are within safe limits. Occasionally, however, fads come along that advocate mega-doses as the latest cure-all. But more isn't always better. In fact, mega-doses can cause more problems than they claim to cure. For healthy adults, here are the latest safe upper limits for many nutrients established by the Institute of Medicine at the National Academy of Sciences. (If you have medical problems, discuss the vitamins and other supplements you take with your doctor; these limits may not be safe for your situation.)


Upper Limit*
per Day

Critical Adverse Effect(s)



2,500 mg

Hypercalcemia (a disorder in which the level of calcium in the blood is too high; the most common symptoms are feeling tired, difficulty thinking clearly, lack of appetite, pain, frequent urination, increased thirst, constipation, nausea, and vomiting)


(folic acid)

1,000 ug

Can mask the presence of anemia caused by Vitamin B12 deficiency



350 mg

Diarrhea, abdominal pain, loss of appetite and irregular heartbeat



35 mg

Flushing, itching, headaches, cramps and nausea; more serious adverse effects can include liver damage, high blood sugar and irregular heartbeats.



400 ug

Selenosis (symptoms include gastrointestinal upsets, hair loss, white blotchy nails, and mild nerve damage.)


Vitamin B6

100 mg

Nerve damage in the arms and legs that is usually reversible when supplementation is stopped


Vitamin C

2,000 mg

Diarrhea and abdominal pain


Vitamin D

50 ug

Hypercalcemia (see Calcium above)


Vitamin E

1,000 mg

Bleeding, especially for people on blood-thinning medications, and gastrointestinal complaints



40 mg

Reduced "good" high-density lipoprotein (HDL) cholesterol, suppressed immune system function, diarrhea, heartburn, nausea, vomiting and abdominal pain


*mg = milligram; 1,000 mg = 1 gram
ug or mcg = microgram; 1,000 ug or mcg = 1 mg;
      1,000,000 ug or mcg = 1 gram

Source of Upper Daily Limit: "Dietary Reference Intakes; Applications in Dietary Assessment," Institute of Medicine, National Academy of Sciences, 2000

For more information about vitamins, the Harvard School of Public Health has an excellent article. To read it, click on "Vitamins."

The Memorial Sloan-Kettering Cancer Center is also an excellent source for information about herbs, botanicals and other dietary supplements. Their special Web site contains information about 140 products. To visit it, simply click on Information Resource.