Which individuals should avoid vitamin c supplements because of above-normal risk of negative effects? ?

Excess of everything is bad. This rule also applies to the level of vitamins in our body. For example, the high level of Vitamin C in body is not good and can result in some adverse effect.

Vitamin C is a water soluble vitamin which means it doesn't stay in body and excrete with water.

People who take supplements with high level of vitamin C can face digestive distress and kidney stones. It is because the excess level of vitamin C starts accumulating in body. Excess vitamin C in body can cause diarrhea and nausea and also kidney stones.

So people who have kidney stones and such related conditions should not take vitamin C.