How overdosing on vitamins and minerals can lead to health problems

They're sold over the counter, right next to the toothpaste. So vitamin and mineral supplements must be perfectly safe, right? Not so fast. Even though smart people you know take them, these supplements can be dangerous in certain dosages and circumstances. Here are some of the risks you face when you overdose on vitamins and minerals.

Yes. Sadly, it's all too easy to take "megadoses" of certain vitamins and minerals.

• Some supplement manufacturers try to impress consumers with a "more is better" approach. You can easily find multivitamins or single-nutrient supplements that contain doses close to or even above Tolerable Upper Intake Levels.
• In recent years, new data has brought shocking revelations about supplements once considered extremely safe.
• None was quite as jarring as the 2004 study analysis involving 136,000 subjects. They found that subjects who took daily supplements of 400 to 800 IU of vitamin E — the amount you find in most over-the-counter formulations — had a higher risk of premature death than those who took placebos.
• Since vitamin E was the darling supplement of the 1990s, with dozens of small studies suggesting that it offered benefits such as preventing heart disease and dementia, this finding sent shockwaves through the health care community.
• Not long after this, researchers reported that using antioxidant formulations containing beta-carotene, vitamin A and vitamin E not only offered study subjects no tangible benefits, but also slightly increased their chances of death. (Vitamin C takers also saw no benefits, though they suffered no harm.)

Below are the daily Tolerable Upper Intake Levels (or ULs) of several common nutrients. These are not the recommended daily doses but the maximums you should never exceed.

• Vitamin A (15,000 IU): If you exceed the UL, you may experience nausea, vomiting, blurred vision, headaches, reduced bone density, birth defects.
• Vitamin B3 (niacin) (35 milligrams): If you exceed the UL, you may experience flushing, rashes, nausea, liver damage.
• Vitamin B6 (100 IU): If you exceed the UL, you may experience numbness in hands and feet, nerve damage.
• Vitamin B9 (folic acid) (1,000 micrograms): Exceeding the UL could mask a vitamin B12 deficiency until irreversible neurological changes occur.
• Vitamin C (2,000 milligrams): If you exceed the UL, you may experience nausea, kidney stones, increased absorption of iron.
• Vitamin D (2,000 IU): If you exceed the UL, you may experience nausea, vomiting, constipation, weakness; effects can include heart rhythm abnormalities, mental confusion and kidney damage.
• Vitamin E (1,500 IU): If you exceed the UL, you may experience dizziness, fatigue, headache, weakness, blurred vision. May interfere with absorption of other vitamins; may cause internal bleeding or stroke if mixed with blood-thinning medications. Taking daily doses of vitamin E as low as 400 IU may cause increased risk of premature death.
• Calcium (2,500 milligrams): If you exceed the UL, you may experience decreased absorption of other minerals, kidney stones, impaired kidney function.
• Iron (45 milligrams): If you exceed the UL, you may experience over time, iron can accumulate in the body, possibly causing cirrhosis or cancer of the liver or damage to the heart and pancreas.
• Magnesium (350 milligrams): If you exceed the UL, you may experience nausea, confusion, diarrhea, abdominal cramps, muscle weakness, low blood pressure, irregular heartbeat, kidney damage.
• Selenium (400 micrograms): If you exceed the UL, you may experience nausea, vomiting, diarrhea, hair loss, nail brittleness, numbness in hands and feet.
• Zinc (40 milligrams): If you exceed the UL, you may experience although usually seen only with consumption of extremely large doses — 300 milligrams to one gram a day — too much zinc can cause nausea, vomiting, diarrhea, headaches and impaired immune system function.

Keep these findings in mind when reading about the potential benefits of the new "hot" vitamin or mineral supplement making headlines. As more research is done, we may again see today's super-nutrient become tomorrow's killer pill.