The Multivitamin's Biggest Trial Came Back Null. Its Grade Is C+.

A multivitamin is the most ordinary supplement in the library and one of the most searched, at about 165,000 queries a month. It is usually sold as insurance: a daily pill that covers dietary gaps so nothing important runs low. Insurance is a testable promise, and the multivitamin is one of the few supplements with a trial big enough to test it.

What COSMOS found

COSMOS, the biggest randomized data behind the multivitamin, assigned older adults to a daily multivitamin or placebo. On the headline outcomes the result was null: no benefit for total cancer, no benefit for major cardiovascular events, and no benefit for mortality. A separate cohort analysis of about 390,000 people, published in 2024, found the same on the outcome that matters most: no mortality benefit from daily multivitamin use.

That is the main finding, and this piece does not bury it. For a healthy adult eating a reasonably varied diet, the biggest and best data say the daily multivitamin does not prevent the diseases it is implicitly sold against.

The two signals that keep the grade at C+

A null headline is not the whole file, and a fair grade has to carry what else the record shows.

First, cognition. COSMOS sub-studies in older adults reported modest benefits on global cognition and episodic memory, strongest in participants with cardiovascular disease. These are secondary analyses, not the trial's primary outcomes, and the effects are small. They are also real randomized data, which is more than most supplement claims have.

Second, aging markers. A 2026 COSMOS biomarker analysis in Nature Medicine found roughly four months less epigenetic aging over two years on the multivitamin arm versus placebo. Epigenetic clocks are surrogates: they move without proof that health or lifespan moves with them. Howard Sesso, COSMOS's principal investigator, presented the finding as promising but preliminary, and has stressed learning what benefits actually exist before recommending use. That caution is the correct read of his own data.

A January 2026 meta-analysis added one more weak signal, for colorectal cancer, alongside no benefit for all-cause mortality or other cancers. Weak signals at the edge of a null record do not change the headline. They explain why the grade is C+ rather than lower.

What the grade means at the shelf

At $5 to $15 a month, a multivitamin is cheap, and its worth-it grade is still only C+, because our worth-it model caps any record below the evidence floor however inexpensive it is. Cheap does not rescue thin. For a reader with a documented deficiency, the answer is different and specific: test, then take the nutrient that is low, at a graded dose. For everyone else, the honest summary of the largest evidence base in the category is this: probably harmless at normal doses, helpful in two narrow lanes that may not apply to you, and not the insurance policy on the label.

Grades are research summaries, not medical advice.