I like to take vitamin C whenever I'm cold. It's also known as ascorbic acid
How does vitamin C help when you are cold?
It's usually shortening the length of my colds! There is scientific evidence that's what it helps with.
Is it possible to take too much vitamin C?
Vitamin C
Vitamin C, also known as ascorbic acid and -ascorbic acid, is a vitamin found in various foods and sold as a dietary supplement. It is used to prevent and treat scurvy. Vitamin C is an essential nutrient involved in the repair of tissue and the enzymatic production of certain neurotransmitters. It is required for the functioning of several enzymes and is important for immune system function. It also functions as an antioxidant. 
Current evidence does not support its use for the prevention of the common cold. There is
Vitamin C megadosage
Vitamin C megadosage is a term describing the consumption or injection of vitamin C (ascorbic acid) in doses well beyond the current Recommended Dietary Allowance of 90 mg/day, and often well beyond the Tolerable upper intake level of 2,000 mg/day. There is no scientific evidence that megadosage helps to cure or prevent cancer, the common cold, or a variety of other medical conditions.
Historic advocates of vitamin C megadosage include Linus Pauling, who won the Nobel Prize in Chemistry in 1954. Pauling