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.

Methylated forms (methylfolate, methylcobalamin, pyridoxal-5-phosphate) are the bioactive versions your body can use directly without enzymatic conversion. Approximately 40% of the population carries MTHFR gene variants that reduce the ability to convert synthetic folic acid into its active form. If you are over 50, have known MTHFR polymorphisms, or want the most readily usable forms, a methylated multivitamin is a better choice. For most healthy young adults, standard forms used in major clinical trials are effective.

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.

Nausea is the most common multivitamin side effect and is almost always caused by taking the supplement on an empty stomach. Iron and zinc are the primary irritants. Solutions: always take your multivitamin with a meal containing some fat (also improves absorption of fat-soluble vitamins A, D, E, K), switch to an iron-free formula if you do not need supplemental iron, or try a whole food-based multivitamin which tends to be gentler on the stomach.

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. Certain medications also deplete key nutrients — for example, proton pump inhibitors significantly increase the risk of vitamin B12 deficiency with long-term use ^[1]. A multivitamin provides insurance against these gaps, though it should complement — not replace — a nutrient-dense diet.