Which Supplements Actually Have Strong Evidence? A Tiered Guide

Some supplements have decades of rigorous research behind them, while others rest on little more than promising early studies. This guide uses evidence ratings drawn directly from the NIH Office of Dietary Supplements to help you tell the difference.

Please note: supplements are not intended to diagnose, treat, cure, or prevent any disease, and these statements have not been evaluated by the FDA. For personal dosing decisions or questions about combining supplements with medications, consult a clinician or pharmacist.

How We Grade Evidence

Each claim below is assigned one of four ratings that mirror the language used by the NIH Office of Dietary Supplements.

• Strong: Consistent, high-quality evidence from multiple well-designed studies.
• Moderate: Generally supportive evidence, but with some limitations or inconsistency.
• Limited: Early or mixed findings that do not yet support a firm conclusion.
• Insufficient: Too little reliable evidence to draw any meaningful conclusion.

Multivitamins and Multiminerals

What they are

Multivitamin and multimineral supplements, often called MVMs, combine a range of vitamins and minerals in a single product. No single adult target intake has been established for an MVM as a whole, because the relevant reference values apply to each individual nutrient it contains.

What the evidence supports

• Taking an MVM increases total intake of vitamins and minerals and helps people reach recommended amounts when diet alone falls short. [Strong]
• Periconceptional folic acid, commonly obtained through a prenatal MVM, is associated with a reduced risk of neural tube defects. [Strong]
• Prenatal MVMs may improve birth outcomes such as low birth weight and preterm birth compared with iron and folic acid alone. [Moderate]
• MVMs helping to maintain or enhance cognition in older adults. [Limited]
• MVMs preventing cancer overall, cardiovascular disease, or reducing all-cause mortality. [Insufficient]

One specific exception worth noting is the AREDS formula, a high-dose antioxidant plus zinc combination studied for eye health. Research suggests it may slow progression from intermediate to advanced age-related macular degeneration. [Moderate] This is a targeted formula, not a standard daily MVM.

Safety and interactions

A basic MVM that provides nutrients at roughly recommended amounts is considered safe for most healthy adults, according to the NIH Office of Dietary Supplements. The main safety concern is cumulative excess: using an MVM alongside other supplements and fortified foods, or choosing a high-potency product, can push individual nutrient intakes above safe levels.

The most clinically important drug interaction involves vitamin K. Vitamin K found in many MVMs can reduce the effectiveness of warfarin (Coumadin, Jantoven) and similar anticoagulants. Anyone taking a blood thinner should speak with their provider before starting any MVM. Additionally, proton pump inhibitors and other acid-reducing medications can decrease absorption of food-bound vitamin B12, which is relevant for people relying on an MVM as their primary B12 source.

Vitamin D

What it is

Vitamin D is a fat-soluble nutrient the body can synthesize through sun exposure and also obtains from certain foods and supplements. The NIH Office of Dietary Supplements sets the adult Recommended Dietary Allowance at 15 mcg (600 IU) per day, rising to 20 mcg (800 IU) for adults aged 71 and older. The Tolerable Upper Intake Level from all sources combined is 100 mcg (4,000 IU) per day for everyone aged 9 and older, including during pregnancy and lactation.

What the evidence supports

• Vitamin D promotes intestinal calcium absorption and maintains serum calcium and phosphate concentrations needed for normal bone mineralization. [Strong]
• Vitamin D deficiency causes rickets in children and osteomalacia in adults. [Strong]
• Vitamin D combined with calcium supplements can slightly increase bone mineral density in some postmenopausal women and older men and may reduce fracture rates in institutionalized older adults. [Moderate]
• Vitamin D supplements reducing falls or fractures in community-dwelling older adults. [Limited]
• Vitamin D supplements preventing or treating cancer, cardiovascular disease, depression, multiple sclerosis, type 2 diabetes, or aiding weight loss. [Insufficient]

Safety and interactions

Staying within the 100 mcg (4,000 IU) upper limit avoids the risk of hypercalcemia. A health care provider may prescribe higher doses for diagnosed deficiency, but self-directed high-dose supplementation is not advisable. Notable interactions include orlistat, which can reduce vitamin D absorption; statins, which may be less effective at high vitamin D doses; corticosteroids, which can lower vitamin D blood levels; and thiazide diuretics, which combined with vitamin D supplements can raise blood calcium to problematic levels.

Omega-3 Fatty Acids (EPA and DHA)

What they are

Omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). The Institute of Medicine set an Adequate Intake for ALA of 1.6 g per day for adult men and 1.1 g per day for adult women aged 19 to 50, but did not establish a Recommended Dietary Allowance because data were insufficient to calculate an Estimated Average Requirement. No specific intake target exists for EPA or DHA, and no Tolerable Upper Intake Level has been set for any omega-3.

What the evidence supports

• ALA, EPA, and DHA are structural components of cell membrane phospholipids, with DHA especially concentrated in the retina, brain, and sperm. [Strong]
• ALA is an essential fatty acid that must be obtained from the diet. [Strong]
• Long-chain omega-3 supplements (EPA and DHA) can lower elevated serum triglyceride levels. [Moderate]
• Omega-3 supplements relieving symptoms of rheumatoid arthritis or reducing cardiovascular events in the general population. [Limited]
• Omega-3 supplements preventing cognitive decline, dementia, or Alzheimer disease, improving infant neurodevelopment, or slowing age-related macular degeneration. [Insufficient]

Safety and interactions

Omega-3 supplements are generally regarded as safe. Side effects tend to be mild and may include an unpleasant taste, heartburn, nausea, or loose stools. The FDA advises that supplement labels should not recommend more than 2 g of EPA and DHA combined per day from supplements. Because omega-3s can reduce platelet aggregation, combining them with anticoagulant or antiplatelet medications such as warfarin, aspirin, or clopidogrel may theoretically increase bleeding risk. Clinical trials have generally not shown clinically significant bleeding, but periodic monitoring is advisable. Prescription high-dose omega-3 products used to lower triglycerides are drug-strength doses and should only be taken under medical supervision.

Key Takeaways

• Strong evidence supports specific physiological roles for vitamin D and omega-3 fatty acids, as well as the value of prenatal folic acid.
• Moderate evidence supports some benefits of vitamin D combined with calcium for bone health, omega-3s for elevated triglycerides, and prenatal MVMs for certain birth outcomes.
• Evidence is insufficient to support using any of these supplements to prevent or treat most chronic diseases.
• Drug interactions exist for all three categories. Always talk to a doctor or pharmacist before combining supplements with any medication.