Do I need a multivitamin, or is a balanced diet enough?
Need depends on diet, age, pregnancy, absorption, medicines and demonstrated deficiencies, not tiredness alone. A suitable supplement can fill specific gaps, but cannot replace varied food or guarantee prevention of cancer or heart disease.
Details
Identify the actual gap
A vegan diet needs attention to vitamin B12; pregnancy needs an appropriate supplementation plan; some conditions and operations impair absorption. These do not mean any multivitamin will be adequate. Review diet and symptoms before indiscriminate testing.
More is not better
Avoid overlapping products containing the same nutrients. High doses can harm, vitamin K interacts with warfarin, and iron or calcium can affect absorption of some medicines. Keep iron-containing supplements away from children.
Choose with a purpose
Review ingredients, dose and goals with a pharmacist or clinician, especially during pregnancy or chronic illness. Persistent fatigue, weight loss or new symptoms need evaluation. Food also supplies fibre and other components a tablet cannot replace.
Related reading: What are antioxidants, and do supplements prevent disease?.