How much is too much vitamin C?

The correct dosage of Vitamin C differs, depending on your age, gender and daily habits.  According to the American Dietetics Association, Institute of Medicine and the Food and Nutrition Board, the DRI (Dietary Reference Intake) of Vitamin C is 90mg for an adult male, and 75mg for an adult female.  If you are a smoker, you must add 35mg to this DRI.  Smoking essentially fills your body with something known as "free radicals", which work on destroying any cell they hit, and sometimes alter the DNA within the cell (hence cancer).  Vitamin C works as an antioxidant, which helps eliminate free radicals.

Many foods are naturally high in Vitamin C, with the highest being red peppers, then oranges, then kiwi fruit, and so on.  Vitamin C deficiencies are rare in the United States, and most developed countries.  As of right now, most studies show that there is no evidence to support Vitamin C being used to treat cancer, but there are still studies being conducted. 

Vitamin C has a very low toxicity level, which allows it to be taken in mega-doses (anything excessive, such as 1000-2000mg/dose).  This can cause gastrointestinal symptoms in some people, which happens when your body does not absorb the high levels properly.  Some studies even suggest that 1000mg or more may produce kidney stones, but there are conflicting finding on this matter. 

Basically, you want to do what your body is programmed for.  If you are eating a diet rich in fruits, vegetables, lean meats, whole grains and nuts, then you will get the amount of Vitamin C you need. Taking supplements (such as Emergen-C) should be used when you have a cold, or are around others who are sick, but not on a daily bases.  Some people take mega doses due to the antihistamine affects on allergies, but this should be done under a doctor's supervision only.