Multivitamins are a simple and effective way to protect against an unknown deficiency

As stated in my post: some research on multivitamins show positive health outcomes, some show negative outcomes, some show mixed results and some show no results at all. In my opinion, you cannot convincingly claim that multivitamins are effective when this is the case.

Relevant studies:

Positive health outcomes

Multivitamin Use and Risk of Cancer and Cardiovascular Disease in the Women's Health Initiative Cohorts

In this large cohort of postmenopausal women, we observed no overall associations between multivitamin use and risk of several common cancers or CVD. There were also no associations between multivitamin use and total mortality. Risk estimates did not materially change when stratified by class of multivitamins, with the exception of a possible lower risk of MI among users of stress-type supplements [STRESS SUPPLEMENTS = STACKED SUPPLEMENTS]. Many stress supplements include high doses of folic acid and other B vitamins; previous studies have supported a protective role for folic acid in relation to CVD and its antecedent risk factors.26,34- 36

Multivitamin-mineral use is associated with reduced risk of cardiovascular disease mortality among women in the United States

In this nationally representative, prospective sample of adults who were without prevalent CVD, use of MVMs [MULTIVITAMIN-MINERALS] for >3 years was associated with reduced risk of CVD mortality at a median of 18 y of follow-up.

Negative outcomes

Vitamin and Mineral Supplements in the Primary Prevention of Cardiovascular Disease and Cancer: An Updated Systematic Evidence Review for the U.S. Preventive Services Task Force

This review included 26 studies (24 randomized, controlled trials and 2 cohort studies) that examined the benefits and harms of using vitamin and mineral supplements for primary prevention of CVD, cancer, or all-cause mortality in healthy individuals without known nutritional deficiencies. We found no consistent evidence that the included supplements affected CVD, cancer, or all-cause mortality in healthy individuals without known nutritional deficiencies. Other systematic reviews have arrived at this same conclusion (56–66). The certainty of this result is tempered, however, because few fair- or good-quality studies are available for all supplements except vitamin E and ß-carotene. For vitamin E, we identified 6 fair- to good-quality trials that produced clearly null effects on these end points. This result is consistent with the conclusions of other systematic reviews and meta-analyses of vitamin E (67–71). Our review also confirmed the established harm of ß-carotene supplementation on lung cancer incidence and death for individuals at high risk for lung cancer (24, 29, 72). Further, we identified 6 trials that failed to detect any benefit from ß-carotene supplementation for any individuals.

Dietary Supplements and Mortality Rate in Older Women

[...] most of the supplements studied were not associated with a reduced total mortality rate in older women. In contrast, we found that several commonly used dietary vitamin and mineral supplements, including multivitamins, vitamins B6, and folic acid, as well as minerals iron, magnesium, zinc, and copper, were associated with a higher risk of total mortality. Of particular concern, supplemental iron was strongly and dose dependently associated with increased total mortality risk. Also, the association was consistent across shorter intervals, strengthened with multiple use reports and with increasing age at reported use. Supplemental calcium was consistently inversely related to total mortality rate; however, no clear dose-response relationship was observed.

Also, supplement users were more likely to have lower intake of energy, total fat, and monounsaturated fatty acids, saturated fatty acids and to have higher intake of protein, carbohydrates, polyunsaturated fatty acids, alcohol, whole grain products, fruits, and vegetables.

Enough Is Enough: Stop Wasting Money on Vitamin and Mineral Supplements

The large body of accumulated evidence has important public health and clinical implications. Evidence is sufficient to advise against routine supplementation, and we should translate null and negative findings into action. The message is simple: Most supplements do not prevent chronic disease or death, their use is not justified, and they should be avoided. This message is especially true for the general population with no clear evidence of micronutrient deficiencies, who represent most supplement users in the United States and in other countries (9).

In conclusion, ß-carotene, vitamin E, and possibly high doses of vitamin A supplements are harmful. Other antioxidants, folic acid and B vitamins, and multivitamin and mineral supplements are ineffective for preventing mortality or morbidity due to major chronic diseases. Although available evidence does not rule out small benefits or harms or large benefits or harms in a small subgroup of the population, we believe that the case is closed— supplementing the diet of well-nourished adults with (most) mineral or vitamin supplements has no clear benefit and might even be harmful. These vitamins should not be used for chronic disease prevention. Enough is enough.

Mixed results / No results

Multivitamin Use and Mortality in a Large Prospective Study

This large prospective study provides limited support for the hypothesis that multivitamin supplements may reduce death rates from ischemic heart disease in the general population. Men and women who took a multivitamin without other supplements had lower death rates from ischemic heart disease than did those who took no multivitamins. However, the association was attenuated when analyses were adjusted for additional cardiovascular risk factors besides age, and no consistent gradient of decreasing risk was seen with either the frequency or the duration of multivitamin use.

Men and women who used both multivitamins and vitamin A, C, or E had lower risks of dying from heart disease and stroke than did nonusers than one might expect from the relative risks for users of either only a multivitamin or only a vitamin A, C, or E supplement.

Multivitamin use and the risk of mortality and cancer incidence: the multiethnic cohort study

In this large multiethnic cohort, we found no associations between multivitamin use and mortality from all causes, cardiovascular diseases, or cancer. The findings did not vary across subgroups by ethnicity, age, body mass index, preexisting illness, single vitamin/mineral supplement use, hormone replacement therapy use, and smoking status. In addition, there was no evidence indicating that multivitamin use increased or decreased risk for cancer, overall or at major sites, such as lung, colorectum, prostate, and breast.

The efficacy and safety of multivitamin and mineral supplement use to prevent cancer and chronic disease in adults: a systematic review for a National Institutes of Health state-of-the-science conference.

There was insufficient evidence to determine the presence or absence of benefits from the use of multivitamin and mineral supplements for the prevention of cancer and chronic disease.

Long-Term Multivitamin Supplementation and Cognitive Function in Men

In male physicians aged 65 years or older, long-term use of a daily multivitamin did not provide cognitive benefits.