You can take too high a level of vitamins and minerals but this is true of many things including salt, alcohol and even water. All the major vitamins and minerals have established safety levels (see HSIS web site). Water-soluble vitamins are usually very safe whereas some of the fat-soluble vitamins can only be taken safely for long periods of time at levels only 2 or 3 times the Recommended Daily Allowance. This is because excesses of these vitamins are stored in the body and can accumulate.