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A scoping review of multivitamin trials found that supplements most consistently raised circulating folate and vitamins B6 and B12. Reported benefits for pregnancy outcomes and cognitive tests varied, and the review did not establish that higher nutrient levels generally improve health. Its authors did not assess study bias or evidence certainty, or conduct a meta-analysis.
A review of clinical trials found that multivitamins most consistently raise blood levels of folate and vitamins B6 and B12, but the evidence does not show that those changes reliably lead to better health outcomes. The review, published in the Journal of Dietary Supplements, also found varied results for pregnancy-related outcomes and cognitive tests, with potential effects differing by formulation, population and duration of use.
The researchers included 157 publications: 103 primary intervention reports and 54 related papers, some based on the same trial populations. Their search covered the Cochrane Library, Scopus and PubMed through September 9, 2026. Eligible studies examined oral tablets or capsules containing at least 14 vitamins and minerals, each meeting specified minimum nutrient amounts and upper-intake limits. Participants generally were adults, with an exception for some prenatal studies in low- and middle-income countries.
Across the studies, blood tests most consistently showed higher circulating folate, vitamin B6 and vitamin B12 after supplementation. Mineral responses were less consistent: findings for iron, zinc and iodine depended on circumstances, including participants’ starting nutrient levels and clinical setting. Some studies reported pregnancy-related or cognitive benefits, but the results differed across groups and outcomes. The review said that higher nutrient levels did not consistently translate into improvements in clinical outcomes.
The authors also described safety findings, while noting that safety reporting differed between trials. Multivitamin use was reported as generally well tolerated in studies involving pregnant participants, older adults and people recovering from surgery. Some products were associated with mild digestive complaints, especially those containing iron or zinc. Specific trial reports included modest increases in rash and nosebleeds in the Physicians’ Health Study II and increased gastrointestinal bleeding in the COSMOS study. A cataract trial reported adverse changes in some lens-opacity subtypes.
Higher Vitamin Levels, Mixed Health Outcomes
The distinction between a change in a blood test and a demonstrated health benefit matters for people who take multivitamins to prevent illness or protect memory and aging. The review supports the conclusion that some formulations can raise certain nutrient levels; it does not establish that taking a multivitamin will improve health for every person, or that more of those vitamins will produce a meaningful clinical benefit.
That uncertainty is relevant because about one-third of U.S. adults take multivitamins, according to the review, and use rises with age. Products differ in ingredients and doses, while people differ in diet, baseline nutrient levels and health circumstances. The findings therefore do not support treating all multivitamins as interchangeable or assuming that results from a specific trial apply to every product or population.
Safety also depends on the individual and the formulation. The review notes that vitamin K can interfere with anticoagulants such as warfarin. People taking those medicines should consult a qualified healthcare professional before using multivitamins. The trial reports of bleeding and other adverse effects are not proof that every product carries the same risks, but they show why benefits and harms need to be considered alongside nutrient changes.
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How the Trial Evidence Was Reviewed
The review set out to map clinical trial evidence on the efficacy and safety of broad-spectrum multivitamins in adults. Researchers searched three databases for peer-reviewed English-language studies. Eligible trials had to include more than 10 people and use an oral multivitamin for at least two weeks. The products had to contain at least two vitamins and two minerals, as well as at least 14 vitamins and minerals overall.
Of 5,263 records identified, 1,537 duplicates were removed. After screening and citation searches, the researchers included 157 publications. Most studies were conducted in North America, Europe and Asia. The review also included selected prenatal research from low- and middle-income countries, including some participants under 18.
This was a scoping review, designed to summarize the range of available evidence rather than calculate a single combined effect. The researchers did not perform a meta-analysis and did not assess risk of bias or certainty of evidence. The review consequently describes reported patterns across trials, but cannot establish a precise overall effect or how dependable each reported benefit is.
“Improvements in circulating folate, vitamin B6, and vitamin B12 were most consistent.”
— The review authors
What the Review Cannot Establish
The review does not determine whether multivitamins improve overall health, prevent disease or extend life. Because the authors did not assess risk of bias or evidence certainty and did not combine results in a meta-analysis, the strength and size of reported effects remain uncertain. Some publications also drew on the same trial populations, so the publication count does not represent 157 wholly separate trials.
Findings varied with product, population, duration and participants’ starting nutrient status. The available report does not establish which people without a known deficiency benefit from supplementation, which formulation or dose would be appropriate for a particular person, or whether biochemical changes persist over time. Safety reporting was inconsistent, leaving some adverse effects harder to compare across studies.
Some individual results also require careful limits. For example, a two-week study in women with preeclampsia examined multivitamins added to intravenous magnesium sulfate; its findings do not establish prevention of preeclampsia or general benefit in uncomplicated pregnancies. Other trial results described in the review concern particular outcomes and cannot by themselves establish broad health effects.
What Future Trials Need to Clarify
The review is an evidence map, not the announcement of a new clinical trial or a change to supplementation guidance. The researchers said that understanding how products vary by market, manufacturing period and life stage could help guide more targeted strategies. Further research would need to examine defined formulations and populations, measure meaningful health outcomes as well as nutrient levels, and report adverse events consistently.
For now, the review offers no single answer for every multivitamin user. Decisions about supplementation should take account of a person’s circumstances and medicines; those using warfarin or other anticoagulants should seek guidance from a qualified healthcare professional. The next step in the evidence base is clearer, more comparable trial research that can determine when observed changes in vitamin levels translate into benefits that matter to patients.
Key Questions
Which vitamin levels rose most consistently?
The review found the most consistent increases in circulating folate and vitamins B6 and B12. Results for other nutrients varied among studies and participants.
Did the review show that multivitamins improve health?
No general health benefit was established. Some trials reported pregnancy-related or cognitive outcomes, but results varied, and higher nutrient levels did not consistently correspond to better clinical outcomes.
How strong is the evidence?
The review summarized a broad set of trial publications, but its authors did not assess risk of bias or evidence certainty and did not conduct a meta-analysis. The size and reliability of reported benefits therefore remain unclear.
Can multivitamins cause side effects or interact with medicines?
Some trials reported mild digestive complaints, while individual studies reported other adverse effects. The review also notes that vitamin K may interact with warfarin. People taking anticoagulants should consult a qualified healthcare professional before using a multivitamin.
Does the review give individual supplement advice?
No. It maps trial findings and does not identify one product or dose as suitable for everyone. Supplement decisions depend on individual circumstances and should be discussed with a qualified healthcare professional when medical conditions or medicines are involved.
Source: news-medical.net
