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:
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.
If you are taking:
Vitamin C may have help reduce high blood pressure. A number of studies have been done to test the vitamin for this purpose, and results are a bit mixed.
Vitamin C is an essential nutrient that must be obtained from food or supplements; the body cannot manufacture it. It is present in many foods, including citrus fruits, red chili peppers, sweet peppers, kale, parsley, collards, turnip greens, broccoli, Brussel sprouts, watercress, cauliflower, cabbage, and strawberries. To get therapeutic doses, you will probably need to take a supplement. Because vitamin C is the most popular supplement in the United States, supplements are affordable and widely available.
Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. The daily doses of vitamin C used in these studies were 500 mg, 1,000 mg, and 2,000 mg. One study demonstrated that blood pressure-lowering benefits increased with the larger doses of the vitamin.60 These higher doses are considered safe for adults age 19 and older. (For more information, see Side Effects & Warnings.)
The exact role of vitamin C in lowering blood pressure is not yet clear. Vitamin C is a powerful antioxidant that neutralizes damaging natural substances called free radicals. It works in water, both inside and outside of cells. Antioxidant actions may help treat hypertension by reducing and repairing damage to the lining (endothelium) of blood vessels, enhancing their function and lowering blood pressure.
In a 30-day, double-blind, placebo-controlled study of 39 people taking medications for hypertension, treatment with 500 mg of vitamin C daily reduced blood pressure by about 10%.33 Smaller benefits were seen in studies of people with normal blood pressure or borderline hypertension.34,35 One double-blind study compared 500 mg, 1,000 mg, and 2,000 mg of vitamin C, and found an equivalent level of benefit in all three groups.60
Other studies have failed to find evidence of benefit with vitamin C.36,37,66
Unexpectedly, one study found that a combination of vitamin C (500 mg daily) and grape seed OPCs (1000 mg daily) slightly increased blood pressure.73 Whether this was a fluke of statistics or a real combined effect remains unclear.
Vitamin C may have help reduce high blood pressure. A number of studies have been done to test the vitamin for this purpose.
The exact role of vitamin C in lowering blood pressure is not yet clear. Vitamin C is a powerful antioxidant that neutralizes damaging natural substances called free radicals. It works in water, both inside and outside of cells. Antioxidant actions may help treat hypertension by reducing and repairing damage to the lining (endothelium) of blood vessels, enhancing their function and lowering blood pressure.
Vitamin C supports immune function, but the exact effect of vitamin C on colds and flu is not clear. Vitamin C serves numerous functions in the body, from producing collagen to working as an antioxidant.
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Vitamin C supports immune function, but the exact effect of vitamin C on colds and flu is not clear. Vitamin C serves numerous functions in the body, from producing collagen to working as an antioxidant.
Vitamin C is an essential nutrient that must be obtained from food or supplements; the body cannot manufacture it. Most of us think of orange juice as the quintessential source of vitamin C, but many vegetables are actually even richer sources. Red chili peppers, sweet peppers, kale, parsley, collard greens, and turnip greens are full of vitamin C, as are broccoli, Brussel sprouts, watercress, cauliflower, cabbage, and strawberries. (Oranges and other citrus fruits are good sources, too.)
One great advantage of getting vitamin C from foods rather than from supplements is that you will get many other potentially healthful nutrients at the same time, such as bioflavonoids and carotenes. However, vitamin C in food is partially destroyed by cooking and exposure to air, so for maximum nutritional benefit you might want to try freshly made salads rather than dishes that require a lot of cooking.
Vitamin C supplements are available in two forms: ascorbic acid and ascorbate. The latter is less intensely sour.
Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. Therapeutic doses such as those used in the research on colds and flu tend to be in the 1,000-2,00 mg range.
Numerous studies have found that vitamin C supplements taken at a dose of 1,000 mg or more daily can reduce the severity cold symptoms and shorten their duration. However, the effect is modest at best.50,51,100,172 In addition, at least one study suggests that vitamin C can enhance the effect of standard cold treatments, such as acetaminophen.109 According to most of these studies, using vitamin C throughout the cold season, rather than only occasionally, appears to be beneficial. For example, a review of 29 placebo-controlled trials involving almost 1,000 episodes of illness concluded that taking at least 2,000 mg per day preventively seems to result in shorter and less severe colds when they occur.172 However, high doses of vitamin C does not appear capable of decreasing the number of colds experienced during a season.
But, many people use vitamin C for colds in a different way: they only begin taking it after cold symptoms first appear. In a review of 7 randomized and non-randomized trials, researchers found that this approach—taking high doses of vitamin C (eg, 2,000 - 4,000 mg) at the first sign of illness—does not seem to affect the cold's severity or duration.172
Although some studies suggest that regular use of vitamin C throughout the cold season can help prevent colds,100,149 most other studies have found little to no benefit along these lines.99
Vitamin C has shown a bit more promise for prevention of one type of cold, the post-marathon sniffle. These are colds that develop after endurance exercise; use of vitamin C before and during competition may help keep you cold-free afterwards.54,55 In addition, vitamin C seems to help prevent respiratory infections among individuals who are actually deficient in the vitamin.56
Vitamin C has received a lot of attention for its potential to help prevent and treat infections like those that cause colds and the flu. Research demonstrates that, while vitamin C may mildly reduce symptoms of colds when they occur, it probably does not help prevent colds.
Vitamin C supports immune function, but the exact effect of vitamin C on colds and flu is not clear. Vitamin C serves numerous functions in the body, from producing collagen to working as an antioxidant.
