https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2820369

With as many as 1 in 3 US adults using multivitamin supplements, the question as to whether these supplements reduce mortality

They used

three large observational cohort studies with nearly 400,000 participants (median age, 62) who were followed for as long as 27 years (mean, 20 years); these studies included data on diet, self-reported multivitamin use, and mortality.

In adjusted analyses, daily multivitamin use was associated with a very small, but significant (4%), higher all-cause mortality risk. (multivariable-adjusted hazard ratio, 1.04; 95% CI, 1.02-1.07)

Results from the current study — casting some doubt on a mortality benefit of multivitamin use — are unlikely to change the feelings of reassurance that many patients gain.