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A review of clinical trials found that multivitamins most consistently raised blood levels of folate and vitamins B6 and B12. Evidence that those changes improve health outcomes was mixed, and the review did not assess study bias or certainty or combine results in a meta-analysis.
A review of 157 reports on multivitamin trials found that supplements most consistently raised blood levels of folate and vitamins B6 and B12, but evidence that those changes improve health outcomes was mixed. Published in the Journal of Dietary Supplements, the review covered adults and selected pregnancy studies, and its authors did not assess the risk of bias or certainty of the evidence.
The researchers searched PubMed, Scopus and the Cochrane Library for peer-reviewed English-language studies through September 9, 2026. Eligible products were oral tablets or capsules providing at least half of the relevant recommended intake or adequate intake for at least 14 vitamins and minerals, without exceeding applicable upper intake limits. Studies had to involve more than 10 people taking a multivitamin for at least two weeks.
After screening, the review included 103 primary intervention reports and 54 related publications; some publications drew on the same trial populations. Blood-test results most consistently showed higher levels of folate and vitamins B6 and B12. Results for other vitamins and minerals varied, with responses to iron, zinc and iodine depending on circumstances such as participants’ initial nutrient levels and clinical setting.
Some selected trials reported changes in pregnancy outcomes or cognitive test performance, but results differed by population and measure. The review found no consistent link between improved nutrient levels and better clinical outcomes overall. It also did not conduct a meta-analysis, so it did not produce a single pooled estimate of benefit.
Higher Vitamin Levels, Mixed Health Results
The findings distinguish a measurable change in nutrient levels from a demonstrated improvement in health. A supplement can raise a vitamin detected in blood without showing that it prevents illness, improves memory or extends life. That distinction matters for people deciding whether routine supplementation is likely to help them, especially when products and users differ.
Pregnancy findings illustrate why broad conclusions are difficult. The review describes trials with different participants, comparisons and endpoints: JiVitA-3 reported fewer low-birth-weight and preterm births with multiple micronutrients than with iron and folic acid, but no significant reduction in infant mortality through six months. SUMMIT reported an 18% lower relative risk of early infant mortality. These are specific trial results, not evidence that every multivitamin improves pregnancy outcomes.
The safety picture also merits attention. The review describes products as generally well tolerated in some studied groups, but reports varied. Mild digestive complaints were associated particularly with products containing iron or zinc; individual trials also reported other adverse events. The findings do not establish that all formulations have the same benefits or risks.
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What the Review Included
The report, covered by News-Medical on October 29, 2026, summarizes a scoping review of clinical trials rather than a new trial testing one product. The review included studies across adults and an exception for prenatal research in low- and middle-income countries that enrolled some participants younger than 18. Most studies took place in North America, Europe or Asia.
Its inclusion rules excluded non-oral preparations, fortified foods, enteral nutrition, lipid-based formulations and delivery forms such as syrups, powders and sprinkles. As a result, the findings concern a defined category of multivitamin tablets and capsules, not every product marketed as a multivitamin or every way of receiving nutrients.
The authors used a structured screening process, with two reviewers screening records and one checking the other’s extracted data. However, they did not assess risk of bias or evidence certainty. The review therefore maps reported findings but cannot establish how reliable each finding is or how large an overall effect might be.
Limits on Benefit and Safety Claims
The review does not establish the size or reliability of any overall health benefit because it did not evaluate risk of bias or certainty and did not pool results. The number of reports also exceeds the number of distinct primary intervention reports, since some publications used the same trial populations. Reported outcomes varied across formulations, participants and follow-up periods.
Safety reporting differed between studies. The source report notes mild digestive complaints, modest increases in rash and nosebleeds in the Physicians’ Health Study II, increased gastrointestinal bleeding in COSMOS, and adverse changes in some lens-opacity subtypes in a cataract trial. These are findings from particular studies; the review does not show that every multivitamin causes these effects. It also notes that vitamin K can interfere with anticoagulants such as warfarin.
For some pregnancy and cognitive outcomes, the available summary does not establish whether findings apply broadly beyond the studied groups. A two-week trial among women with preeclampsia reported improvements in selected measures when multivitamins were added to intravenous magnesium sulfate, but it does not show that supplements prevent preeclampsia or benefit uncomplicated pregnancies.
What Further Trials Need to Clarify
The review does not announce a new trial or a scheduled next milestone. Its findings point to unresolved questions about which formulations help which groups, how baseline nutrient status affects response, and whether changes in blood levels produce lasting clinical benefits. Better comparisons of distinct products and populations would help address those gaps.
For now, the evidence summarized here supports a measured conclusion: multivitamins can raise some nutrient levels, while benefits and risks depend on the product and situation. People taking warfarin or other anticoagulants should consult a healthcare professional before using a multivitamin because of the potential interaction with vitamin K.
Key Questions
What did the review find most consistently?
It found that multivitamins most consistently raised circulating levels of folate and vitamins B6 and B12. Findings for other nutrients varied.Did the review show that multivitamins improve overall health?
No. Some trials reported benefits for selected outcomes, including certain pregnancy measures and cognitive test performance, but results varied. The review found that higher nutrient levels did not consistently translate into better clinical outcomes.How strong is the evidence?
The review included 103 primary intervention reports but did not assess risk of bias or certainty of evidence and did not perform a meta-analysis. Those limits make it difficult to judge the reliability and overall size of reported effects.Were side effects reported?
Some studies reported mild digestive complaints, particularly with products containing iron or zinc. Other individual trials reported events such as rash, nosebleeds, gastrointestinal bleeding or changes in some lens-opacity subtypes; safety reporting varied across studies.Should people taking warfarin check before using a multivitamin?
Yes. The review notes that vitamin K may interfere with anticoagulants such as warfarin. People taking these medicines should consult a healthcare professional before using a multivitamin.Source: news-medical.net
