I have been taking vitamin c and vitamin d together for a year. Is it okay or bad? Is it helpful taking both? Also if I am overdosing how do I know?

It's Okay. Yes, it is okay to take both vitamin C and vitamin D supplements. You should not take more than the recommended amounts listed on the packaging. Too much of either vitamin C or D may cause gastrointestinal discomfort. In some individuals, too much vitamin C is associated with kidney stones. Too much vitamin D can also lead to muscle weakness.