Not all natural substances are safe. Most vitamins however, even in excessive amounts, have no effects or only mild overdose symptoms like nausea, diarrhoea, or vomiting.
More severe vitamin toxicity symptoms are rare but possible, for example:
This list is by no means complete and serves as a guide only. Most research results are still inconclusive and further studies need to be confirmed.
However, it is prudent to be well informed about anything you take. Even though vitamins can prevent or at least reduce the risk of diseases, they are not medicine. They can interact with pharmaceutical drugs and even hinder the activity of a drug. So, if you take supplements, please let your doctor know.
"A wise man ought to realize that health is his most valuable possession."