A massive NIH study of nearly 400,000 adults over 20 years found that daily multivitamin users had a 4% higher mortality risk compared to non-users. The research showed no mortality benefit whatsoever—contradicting the belief that multivitamins serve as health "insurance". Interestingly, multivitamin users typically had healthier lifestyles overall, yet still showed increased risk.
For healthy adults without diagnosed deficiencies, the healthiest nutrients come from food sources, not processed pills. Some specific concerns include potential buildup of excess iron or niacin from daily use. This reinforces that supplementation should be targeted and evidence-based, not indiscriminate.
If you're white you probably still get enough, especially if you consume dairy. If you have dark skin and live in more northern latitudes, there's a higher chance of deficiency.