Multivitamins are the default supplement most adults take. The evidence base is surprisingly weak — multiple large trials show minimal or no effect on health outcomes for healthy adults eating a reasonable diet. Targeted supplementation based on actual deficiencies works better.
Why multivitamins underperform
Most include dozens of nutrients at low doses — enough to prevent gross deficiency but not enough to address actual deficiencies. The few nutrients many adults are deficient in (vitamin D, possibly omega-3, possibly iron in menstruating women, possibly B12 in vegetarians) need higher doses than multivitamins provide.
Conversely, multivitamins provide nutrients most people get plenty of from food (vitamin A, several B vitamins, copper, zinc). Excess provides no benefit and occasionally harm.
What targeted supplementation looks like
Vitamin D3
1000-2000 IU daily for most adults, year-round in UK. Highest-evidence single supplement.
Omega-3 (EPA+DHA)
1-2g daily if you don't eat oily fish 2x weekly. Cardiovascular and inflammatory evidence.
Iron
Only if tested deficient. Self-supplementing without testing can cause GI side effects without benefit, or rarely overload in conditions like hemochromatosis.
B12
Vegan or vegetarian: 50-100 mcg daily or 1000 mcg weekly. Over 50: worth checking — absorption declines with age.
Folate
Women planning pregnancy: 400-800 mcg from 3 months before conception through first trimester. Otherwise food sources sufficient.
Magnesium
If poor sleep, muscle cramps, or constipation: magnesium glycinate 200-400 mg in evening can help. Most adults marginally deficient; supplementation low-risk.
Where multivitamins might still have role
Pregnancy (specifically prenatal multivitamins — different formulation). Severely restrictive diets. Conditions causing malabsorption (Crohn's, celiac). Post-bariatric surgery. Otherwise: rarely worth the cost vs. targeted supplements.
Test before supplementing
Blood test for vitamin D, ferritin, B12, folate (around £40-60 private, or via GP if symptomatic). Direct targeted supplementation at actual gaps. More effective and cheaper long-term than indiscriminate multivitamins.
Most adults don't need a multivitamin. Test what you're actually low in, supplement specifically, and skip the daily multivitamin habit unless it's genuinely the right tool for your situation.