Though vitamins and other micronutrients occurring in foods are generally considered safe, long-term intake of very high doses of certain vitamins can be harmful to health.
The recommended dietary allowances (RDAs) for vitamins and minerals define a reasonable amount of micronutrient intake for healthy individuals depending on age, gender, and other factors such as pregnancy. Sometimes an intake above the RDA helps alleviate certain health problems.
Tolerable upper intake level
As a number of vitamins may produce adverse effects if taken in excess over long periods of time, health authorities have established tolerable upper intake levels for these.
For a comparison of the upper safe levels for total daily intake of vitamins and minerals for adults from European, U.S., and U.K. health authorities see: PDF
Because of the potential for interactions, dietary supplements should not be taken with medication without first talking to an experienced healthcare provider.