Multivitamins — Frequently Asked Questions

The evidence is mixed but increasingly favorable for specific outcomes. The COSMOS-Mind trial (2022, n=2,262) found daily multivitamin use significantly improved cognitive function in older adults over 3 years, equivalent to slowing cognitive aging by roughly 2 years. PHS-II (2012, n=14,641) found an 8% reduction in total cancer incidence over 11 years. Multivitamins are most beneficial for people with dietary gaps, restrictive diets, or nutrient-depleting medications — rather than as a universal health booster for people already eating a nutrient-dense diet.

Multivitamins can interact with several medication classes. Thyroid medications (levothyroxine) should be taken at least 4 hours before your multivitamin, as calcium, iron, and magnesium reduce thyroid hormone absorption. Blood thinners (warfarin) can be affected by vitamin K content — keep intake consistent and inform your doctor. Antibiotics (tetracyclines, fluoroquinolones) should be separated from multivitamins by at least 2 hours due to mineral chelation. Always tell your healthcare provider about all supplements you take.

Even with a well-balanced diet, nutrient gaps are common. NHANES data shows that over 90% of Americans fall short on vitamin D, 45% are inadequate in magnesium, and 40% are low in vitamins A, C, and E from food alone. Soil depletion, food processing, and modern farming practices have reduced the micronutrient density of many foods compared to decades ago. A multivitamin provides insurance against these gaps, though it should complement — not replace — a nutrient-dense diet.