My Dr. likened it to a top class orchestra. Always in harmony and no duff notes.
If you took a multi-vitamin then there would be no adverse effects but if you didn't have a blood test and decided to dose yourself up with specific vitamins and minerals in the hope to treat yourself for deficiencies that you may or may not have then the orchestra would really be in trouble and the sounds would be horrendous because you now have extra players who know nothing about music. Like the orchestra, your body would not be in harmony.
If you feel fit and healthy, nails, hair and skin are in good condition then why would you need extra vitamins that may do you harm? If you are feeling unwell then the place to sort you out is at the surgery. See your Dr. with any symptoms that you feel may be related to vitamin/mineral deficiencies.