Multivitamins are among the most widely used supplements, taken daily by a large share of adults in many countries, generally as insurance against dietary gaps.
The trial evidence for that use in generally well-nourished populations is considerably weaker than the market size suggests.
What the large trials found
Several large randomised controlled trials have examined multivitamin supplementation with outcomes including cardiovascular events, cancer and mortality.
The broad pattern has been that multivitamin supplementation in generally well-nourished adults has not shown clear reductions in these major outcomes. Several major reviews and preventive services bodies have concluded that evidence is insufficient to recommend multivitamins for the prevention of cardiovascular disease or cancer in the general population.
Some trials have found modest effects on specific secondary outcomes, and results have not been consistent across studies.
The reasonable summary is that for somebody eating an adequate diet, a daily multivitamin has not been shown to produce meaningful health benefits, and it also hasn't been shown to cause harm at typical doses.
Why the intuition is wrong
The reasoning behind taking one — nutrients are essential, therefore more is better — doesn't hold, for a few reasons.
Nutrient responses are generally not linear. Correcting a deficiency produces substantial benefit; adding more once sufficiency is reached produces little or none. The relationship looks like a plateau, not a slope.
Nutrients in food come alongside other compounds and in forms that may differ in bioavailability from isolated supplements. Attempts to isolate the beneficial component of a healthy diet into a pill have a poor track record.
And observational associations between high nutrient intake and good health frequently reflect that people eating nutrient-rich diets differ in many other ways. Trials that isolate the supplement remove those confounders and frequently find the effect disappears.
Where supplementation genuinely helps
Targeted supplementation for identified needs has good evidence, and this is quite different from general multivitamin use.
Folic acid before and during early pregnancy. Strong evidence for reducing neural tube defects. Recommended universally for anyone who could become pregnant.
Vitamin B12 for people on vegan diets. B12 is not reliably available from plant sources. Supplementation is necessary, not optional.
Vitamin D where deficiency is likely. Populations with limited sun exposure, darker skin at higher latitudes, older adults, and those who cover extensively. Many countries recommend supplementation during winter months.
Iron for diagnosed deficiency. Effective and should follow testing rather than assumption, since iron overload carries its own risks.
Specific clinical situations. After certain surgeries, with malabsorption conditions, with particular medications that deplete specific nutrients.
The common feature: a specific nutrient, for a specific reason, in a specific population.
The potential harms
Generally small at typical doses, with some genuine cautions.
Fat-soluble vitamins accumulate. Excessive vitamin A intake is associated with harm including liver toxicity and, in pregnancy, birth defects. This is a reason to avoid stacking multiple products containing the same nutrients.
Some trials of high-dose single antioxidants have found unexpected increases in adverse outcomes in specific populations — notably beta-carotene in smokers. That finding is a useful corrective to the assumption that antioxidants are universally beneficial.
Interactions with medications are real. Vitamin K affects anticoagulant therapy. Some supplements affect absorption of others. Anyone on regular medication should mention supplements to their prescriber, and frequently doesn't.
And there's the substitution risk — the possibility that taking a supplement provides a sense of having addressed diet, reducing motivation to change what's actually eaten. Evidence on this is limited and the mechanism is plausible.
Quality and regulation
Supplements are regulated less stringently than medicines in most jurisdictions, typically as food products.
Independent testing has repeatedly found products containing amounts differing from the label, and occasionally containing undeclared ingredients.
Third-party certification schemes exist and provide some assurance about contents. Products carrying independent verification are a reasonable choice where you're taking something regularly.
The reasonable position
If you eat a reasonably varied diet, a daily multivitamin is unlikely to harm you and unlikely to do much. Whether that's worth the cost is a personal judgement.
If you have a specific risk of deficiency — dietary restriction, limited sun exposure, pregnancy, age, a medical condition, a medication — targeted supplementation is genuinely worthwhile and should be specific rather than general.
If you're unsure whether you're deficient in something, testing is available for the nutrients where it's clinically meaningful, and it's a better basis for decisions than guessing.
And the boring conclusion holds: the dietary pattern matters more than any supplement, and no pill has yet been shown to substitute for it.
General information only. Discuss supplements with a qualified healthcare professional, particularly if you are pregnant, taking medication, or managing a health condition.
The specific-population exception
One case where general multivitamin use does have supporting evidence, and it is worth distinguishing from the general population finding: people with genuinely inadequate dietary intake.
That includes some older adults with reduced appetite, people with restricted diets for medical or economic reasons, and people recovering from illness. In those situations a multivitamin is functioning as a correction for inadequacy rather than as insurance for somebody already sufficient.
Which returns to the same principle running through this whole area. Supplementation helps where something is missing. The question is never whether a nutrient matters, but whether you are short of it, and that is a question with an answer.