“Consuming vitamins or other micronutrients (for example, vitamin C) at doses above healthy or recommended levels provides additional measurable health benefits.”

Evidence from most randomized controlled trials suggests that vitamin C supplementation, usually in combination with other micronutrients, does not reduce the risk of chronic diseases like cardiovascular disease (CVD) or cancer. Although the Linxian trial data suggest a possible benefit, overall, the findings from most intervention trials do not provide convincing evidence that vitamin C supplements provide protection against CVD or reduce its morbidity or mortality. In postmenopausal women with diabetes who participated in the Iowa Women's Health Study, supplemental (but not dietary) vitamin C intake (at least 300 mg/day) was significantly associated with a higher risk of dying from CVD.

Several studies of high-dose vitamin C alone or in combination with other drugs in the treatment of cancer have shown no evidence that it can cure cancer. Some studies of IV vitamin C in cancer patients reported improved quality of life and reductions in cancer-related symptoms, but no effect on survival or tumor size.

Compared with placebo, patients who underwent intravenous vitamin C (IVVC) have reduced duration of vasopressor used (SMD 0.26; CI 0.01–0.51; I² = 87.0%, P = .044), mechanical ventilation (SMD −0.29; CI −0.55 to −0.03; I² = 36.8%, P = .031). This study comprehensively and systematically evaluated vitamin C supplementation in the critically ill through a meta-analysis of randomized controlled trials. There is no difference except for patients who underwent vitamin C and had a reduced duration of vasopressor use and mechanical ventilation.

In this study, we aimed to assess the efficacy of adding high-dose intravenous vitamin C (HDIVC) to the regimens for patients with severe COVID-19 disease. We did not find significantly better outcomes in the group who were treated with HDIVC in addition to the main treatment regimen at discharge. Fowler et al. reported in their randomized, double-blind, placebo-controlled, multicenter trial that high doses of vitamin C did not significantly improve organ dysfunction scores in patients with severe sepsis or ARDS.

Mounting evidence indicates that vitamin C has the potential to be a potent anti-cancer agent when administered intravenously and in high doses (high-dose IVC).

For many nutrients, a systematic determination of the effects of high intakes over extended periods of time has not been conducted. When intakes are inadequate, food fortification may be the appropriate choice for some nutrients, while in other situations, when requirements are markedly higher for some population subgroups than for the general population, supplements may be the most appropriate intervention.

In general, population‐based surveys in low‐ and middle‐income countries suggest that dietary intakes greater than the UL are uncommon, but simulations indicate that fortification and supplementation programs could lead to high intakes under certain scenarios. However, in some cases, it may be reasonable to accept a certain proportion of intakes above the UL if (1) the program is likely to address deficiency; and (2) the total benefits associated with reduced deficiency are deemed to be greater than the risks associated with increases in intake above the UL.

A fixed-effects meta-analysis of all seven RCTs revealed no significant association between vitamin C supplementation and cancer (relative risk, 1.00; 95% confidence intervals, 0.95-1.05). Similarly, subgroup meta-analysis by dose of vitamin C administered singly or in combination with other supplements, follow-up period, methodological quality, cancer mortality, gender, smoking status, country, and type of cancer also showed no efficacy of vitamin C supplementation for cancer prevention. In this meta-analysis of seven RCTs, we found no significant association between vitamin C supplementation and cancer risk.

Two RCTs studied the effects of vitamin C, either alone or in combination with other supplements, and found no statistically significant effect. Only the WHI trial reported cardiovascular disease incidence and mortality and all-cause mortality, and it found no effect after 7 years of follow-up.

Vitamin A supplementation likely reduced all-cause mortality, while zinc supplementation decreased the incidence of diarrhea. Stunting and underweight, however, were improved only among children who were provided with LNS, though MMN supplementation also slightly increased length-for-age z-scores. Importantly, many effects of LNS and MNPs held when pooling data from effectiveness studies.

Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects. RDAs for vitamin A are given as retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids, all of which are converted by the body into retinol.

Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects. For the latest information about the DRIs, go to the Health.

Multimicronutrient DSs were used often; they increased intake adequacy of group B vitamins but failed to sufficiently supplement vitamin D, potassium, calcium, and iodine. Although DS use increased micronutrient intake sufficiency when used properly, the knowledge on micronutrient inadequacy in all dietary patterns should be increased and the public should be educated on the proper use of DSs.

Our study demonstrated a significant correlation between vitamin C consumption and death. Vitamin C consumption significantly reduces mortality risk with COVID‐19 patients (OR = 0.54, 95% CI = 0.42–0.69, *p* < 0.00001). Furthermore, there was a link between the severity of COVID‐19 and the intake of vitamin C. Patients who consumed vitamin C showed 0.63 times less severity than those who did not take vitamin C (OR = 0.63, 95% CI = 0.43–0.94, *p* = 0.02).

High-dose vitamin C has been studied as a cancer treatment since the 1970s, but clinical trials have not shown consistent benefits for cancer prevention or treatment with oral supplementation. Intravenous high-dose vitamin C may have anticancer effects in specific contexts, but evidence for broad health benefits beyond recommended levels remains limited.

If an individual's usual nutrient intake remains below the UL, no adverse effects are expected to occur. At habitual intakes above the UL, the risk of adverse effects increases as the level of intake increases. The UL is not a recommended intake.

Tolerable upper intake level (UL): the maximum level of total chronic daily intake of a nutrient (from all dietary sources) which is not expected to pose a risk of adverse effects in the general population.

These opinions present comprehensive evaluations of possible adverse health effects of individual micronutrients at intakes in excess of dietary requirements and, where possible, establish Tolerable Upper Intake Levels (UL) for different population groups. The approach taken was based on the principles of scientific risk assessment.

In a randomized-controlled double-blind placebo-controlled factorial trial involving 14,641 male physicians aged ≥50 years in the United States, about 1245 of them had at least one cardiovascular event in the space of 8 years follow-up. Vitamin C at a dose of 500 mg/day failed to reduce the risk of such CV events and mortality. Most interventional studies involving vitamin C supplementation show no benefit irrespective of the dosage, but deficient plasma levels are associated with adverse clinical outcomes. This entails that vitamin C supplementation may only produce a significant effect when there is a deficiency.

A report published in Critical Reviews in Food Science and Nutrition suggests raising the RDA of vitamin C from the current levels of 75 mg for women and 90 mg for men to an overall 200 mg per day for adults. Beyond preventing the vitamin C deficiency disease of scurvy, researchers say it’s appropriate to seek optimum vitamin C levels that will saturate cells and tissues, pose no risk, and may have significant effects on public health. 'We believe solid research shows the RDA should be increased,' Frei said. 'And the benefit-to-risk ratio is very high. A 200 milligram intake of vitamin C on a daily basis poses absolutely no risk, but there is strong evidence it would provide multiple, substantial health benefits.'

With most micronutrients, there can be health risks when intake regularly exceeds a high amount, and an upper threshold value set by the United States and Canada (the tolerable upper intake level, or UL) has been established. When assuming high dietary intake plus the amount in a standard multiple micronutrient supplement (with 30 mg of iron), only niacin and iron would be expected to slightly exceed the UL. Known risks for this level intake for each nutrient are transient and mild.

A recent analysis of 29 human studies concluded that daily supplements of 500 milligrams of vitamin C significantly reduced blood pressure, both systolic and diastolic, by 3.84 mmHg and 1.48 mmHg, respectively.

The LOVIT-COVID trial (NCT04401150), a large multicenter RCT published in 2024, found that high-dose intravenous vitamin C (50 mg/kg every 6 hours) in critically ill COVID-19 patients increased the risk of death or persistent organ dysfunction at 21 days compared to placebo (44.5% vs. 38.0%; adjusted odds ratio 1.21; 95% CI 1.09 to 1.34; p<0.001). This refutes benefits of high-dose IV vitamin C beyond standard care in this population.