Humans do not synthesize their own Vitamin C internally and are dependent on the dose they receive from their food. The alternative health industry is pushing Vitamin C as the cure for Cold, Cancer and other diseases. There are people that believe large intakes of Vitamin C can shorten the length of a Cold, but there is no real scientific evidence to back this up.
It is said that larger intake of Vitamin C is harmless which is usually true. However there are cases where this could have the opposite effect. These situations are medically referred to as Contraindication. For instance Vitamin C consumption helps with absorption of Iron, but Iron Overload can cause complication in people.
The take away message from all of this is that it should be possible to safely try higher doses of Vitamin C since the current recommendation is only about 50-100mg. However I'd always consult a doctor before trying anything at all.