Hi There
You can get Vitamin C or "Ascorbic Acid" in doses ranging from 10mg to 500mg.
Vitamin C is not made naturally by Humans due to evolution, so there is a dietary need, either from citrus fruits or supplements.
Vitamin C is essential to strengthen Collagen in the arterial walls hence deficiency causes scurvy, Its Anti-oxidant potential has been a hot debate within the scientific community for years.
The two time Nobel prize winner Chemist "Linus Pauling" was a pioneer in the experimentation/science behind Vitamin C. Here are a few links and an excellent lecture clip from the great man himself.
Wikipedia entry :
http://en.wikipedia.org/wiki/Linus_Pauling
Lecture clip :https://www.youtube.com/watch?v=ArkbEv0Q8D8
http://www.thecureforheartdisease.com/owen/Hea rtCureRD.htm
In excess it is just known to cause diarrhoea, in doses say 3-4g per day.
I hope you find these clips interesting :)