Vitamin C Side Effects and Warnings

Safety Issues

The US government has issued recommendations regarding tolerable upper intake levels (ULs) for vitamin C. The UL can be thought of as the highest daily intake over a prolonged time known to pose no risks to most members of a healthy population. The ULs for vitamin C are as follows:

• Children
  • 1-3 years: 400 mg
  • 4-8 years: 650 mg
  • 9-13 years: 1,200 mg

• Males and Females
  • 14-18 years: 1,800 mg
  • 19 years and older: 2,000 mg

• Pregnant Women
  • 18 years old or younger: 1,800 mg
  • 19 years and older: 2,000 mg

• Nursing Women
  • 18 years old or younger: 1,800 mg
  • 19 years and older: 2,000 mg

However, even within the safe intake range for vitamin C, some individuals may develop diarrhea. This side effect will likely go away with continued use of vitamin C, but you might have to cut down your dosage for a while and then gradually build up again.

Concerns have been raised that long-term vitamin C treatment can cause kidney stones . ¹ However, in large-scale observational studies, individuals who consume large amounts of vitamin C have shown either no change or a decreased risk of kidney stone formation. ^{2 3} Still, there may be certain individuals who are particularly at risk for vitamin C-induced kidney stones. ⁴ People with a history of kidney stones and those with kidney failure who have a defect in vitamin C or oxalate metabolism should probably restrict vitamin C intake to approximately 100 mg daily. You should also avoid high-dose vitamin C if you have glucose-6-phosphate dehydrogenase deficiency, iron overload, or a history of intestinal surgery.

Vitamin C supplements increase absorption of iron . ⁵ Since it isn’t good to get more iron than you need, individuals using iron supplements shouldn’t take vitamin C at the same time except under a physician’s supervision.

One study from the 1970s suggests that very high doses of vitamin C (3 g daily) might increase the levels of acetaminophen (such as Tylenol) in the body. ⁶ This could potentially put you at higher risk for acetaminophen toxicity. This interaction is probably relatively unimportant when acetaminophen is taken in single doses for pain and fever, or for a few days during a cold. However, if you use acetaminophen daily or have kidney or liver problems, simultaneous use of high-dose vitamin C is probably not advisable.

Weak evidence suggests that vitamin C, when taken in high doses, might reduce the blood-thinning effects of warfarin (Coumadin) and heparin . ^{7 8 9} As noted above, one study found that vitamin C at a dose of 1 g daily substantially reduced blood levels of the drug indinavir, a protease inhibitor used for the treatment of HIV infection. ¹⁰ Heated disagreement exists regarding whether it is safe or appropriate to combine antioxidants such as vitamin C with standard chemotherapy drugs. The reasoning behind the concern is that some chemotherapy drugs may work in part by creating free radicals that destroy cancer cells, and antioxidants might interfere with this beneficial effect. ¹¹ However, there is no good evidence that antioxidants actually interfere with chemotherapy drugs, but there is growing evidence that they do not. ¹² The maximum safe dosages of vitamin C for people with severe liver or kidney disease have not been determined.

Interactions You Should Know About

If you are taking:

• Aspirin , other anti-inflammatory drugs, or oral contraceptives : You may need more vitamin C.

• Acetaminophen (eg, Tylenol) : The risk of liver damage from high doses of acetaminophen may be increased if you also take large doses of vitamin C.

• Warfarin (Coumadin) or heparin : High-dose vitamin C might reduce their effectiveness.

• Iron supplements: High-dose vitamin C can cause you to absorb too much iron. This is especially a problem for people with diseases that cause them to store too much iron.

• Medications in the nitrate family: Vitamin C may help maintain their effectiveness. Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.

• Protease inhibitors for HIV: High-dose vitamin C may reduce their effectiveness.

• Cancer chemotherapy : Do not use vitamin C except on physician’s advice.