Supplement guides
Know what you take
Plain-English guides to the most common supplements: forms on the label, official daily amounts, when to take them, what to keep apart and who should ask first. Every fact links to its source: NIH, NHS, MedlinePlus or the FDA.
Vitamins
- B-Complex VitaminsA B-complex supplement combines the eight B vitamins: thiamin, riboflavin, niacin, pantothenic acid, B6, biotin, folate and B12. They are water-soluble, and extra amounts leave in urine, except B12, which the liver can store. Niacin, B6 and folic acid each have upper limits for adults.
- BiotinBiotin (vitamin B7) is a B vitamin the body uses to process fats, carbohydrates and protein. The adult Adequate Intake is 30 mcg a day, and deficiency is rare. High-dose biotin can cause falsely high or falsely low lab results, including thyroid and heart tests. Tell your doctor and the lab before blood tests.
- Folate and Folic AcidFolate (vitamin B9) is a vitamin the body uses to make DNA and for cell division. Folic acid is the form in most supplements and fortified foods. Labels list mcg DFE: 400 mcg of folic acid is shown as 680 mcg DFE. The adult upper limit is 1,000 mcg a day of folic acid from supplements and fortified foods.
- MultivitaminsA multivitamin has no standard definition, so the vitamins, minerals and amounts differ from product to product. Basic ones keep most nutrients at or below the Daily Value. Adding single supplements or fortified foods can push some nutrients above safe upper limits. People taking warfarin should ask first about products with vitamin K.
- Vitamin AVitamin A comes in two dietary types: preformed vitamin A (retinol and retinyl esters) from animal foods and many supplements, and provitamin A such as beta-carotene from plants. Labels use mcg RAE. The adult RDA is 900 mcg RAE for men and 700 for women. High doses of preformed vitamin A can cause birth defects.
- Vitamin B12Vitamin B12 is a water-soluble vitamin the body uses to make red blood cells and DNA and to keep the nervous system working. It is found naturally only in animal foods, so vegans rely on fortified foods or supplements. Cyanocobalamin is the most common supplement form. Metformin and acid-reducing medicines can lower B12 absorption.
- Vitamin CVitamin C (ascorbic acid) is a water-soluble vitamin the body cannot make. It uses it to make collagen and as an antioxidant, and it helps absorb plant iron. Supplement ascorbic acid is absorbed as well as vitamin C in food. Absorption falls below 50% above 1 g a day. The adult upper limit is 2,000 mg a day.
- Vitamin DVitamin D is a fat-soluble vitamin the body uses to absorb calcium and keep bones mineralized. Labels list it in mcg, often with IU: 1 mcg equals 40 IU. Fat in the gut helps absorption. The NIH upper limit for adults is 100 mcg (4,000 IU) a day, and too much can raise blood calcium.
- Vitamin EVitamin E is a fat-soluble nutrient, and alpha-tocopherol is the form people need. Labels now list it in mg. Natural d-alpha-tocopherol is twice as active per mg as synthetic dl-alpha-tocopherol. The adult RDA is 15 mg. High-dose supplements can raise bleeding risk, especially with blood thinners such as warfarin.
- Vitamin K (K1 and K2)Vitamin K is a fat-soluble vitamin the body needs to make proteins for blood clotting and bone metabolism. K1 (phylloquinone) comes mainly from leafy greens; K2 (menaquinones such as MK-4 and MK-7) comes from fermented and animal foods. If you take warfarin, keep vitamin K intake steady and ask your pharmacist before taking a supplement.
Minerals
- CalciumCalcium is the mineral that builds bones and teeth and helps muscles, nerves and blood clotting work. Calcium carbonate absorbs best with food; calcium citrate can be taken without food. The body absorbs the most from doses of 500 mg or less. Experts suggest taking calcium and iron supplements at different times of day.
- IodineIodine is a trace mineral the thyroid uses to make the hormones T4 and T3. US adults need 150 mcg a day, 220 mcg in pregnancy and 290 mcg while breastfeeding. Seafood, seaweed, dairy, eggs and iodized salt supply it. Too much can also disturb the thyroid; the adult upper limit is 1,100 mcg a day.
- IronIron is a mineral the body uses to make hemoglobin, which carries oxygen in red blood cells. Labels list elemental iron, not the compound weight. Vitamin C helps absorb plant iron, while calcium, tea and coffee can reduce absorption. The adult upper limit is 45 mg a day. Keep iron away from children: overdose can be fatal.
- MagnesiumMagnesium is a mineral the body uses in more than 300 enzyme systems, including muscle and nerve function. Supplements come as oxide, citrate, chloride and other forms; forms that dissolve well in liquid are absorbed better. The NIH upper limit of 350 mg a day counts supplements and medicines only, not food. High doses can cause diarrhea.
- PotassiumPotassium is a mineral every cell uses for fluid balance and the electrical gradients nerves and muscles need. Most US supplements contain 99 mg or less, about 2% of the 4,700 mg Daily Value, because of FDA rules on higher-dose potassium drugs. With kidney disease or some blood pressure medicines, ask a doctor first.
- SeleniumSelenium is a trace mineral the body builds into about 25 selenoproteins used in thyroid hormone metabolism, DNA synthesis and protection from oxidative damage. The adult RDA is 55 mcg, and most Americans get enough from food. One Brazil nut can hold 68 to 91 mcg; the adult upper limit is 400 mcg a day.
- ZincZinc is a mineral the body needs for hundreds of enzymes, protein and DNA synthesis, wound healing and immune function. Labels list elemental zinc. Citrate and gluconate are absorbed similarly, and slightly better than oxide. The adult upper limit is 40 mg a day; 50 mg or more for weeks can lower copper.
Herbs, probiotics and other supplements
- AshwagandhaAshwagandha (Withania somnifera) is an herb from Ayurvedic medicine; most supplements use root or root-and-leaf extracts. It appears well tolerated for up to about 3 months, but long-term safety is unknown. Rare cases of liver injury have been reported. Avoid it in pregnancy and breastfeeding, and ask first if you have thyroid or autoimmune conditions.
- BerberineBerberine is a plant compound found in barberry, goldenseal, goldthread, Oregon grape and tree turmeric. It is studied for blood sugar and cholesterol, but NIH's NCCIH calls the evidence weak. It may interact with medicines, and reviews flag liver enzymes that process many drugs. NCCIH says not to use it in pregnancy, breastfeeding or infants.
- Coenzyme Q10 (CoQ10)Coenzyme Q10 (CoQ10) is a compound your body makes and uses to produce energy in cells and as an antioxidant. Levels are highest in the heart, liver, kidneys and pancreas and fall with age. It is fat-soluble and absorbed best with fat. It may interact with warfarin and insulin, so ask first.
- CollagenCollagen is the most abundant protein in the body and a main part of skin, bone, tendons and cartilage. Supplements usually contain hydrolyzed collagen (collagen peptides) from cow, pig or fish sources. Collagen lacks the amino acid tryptophan, so it is an incomplete protein. Supplements are not FDA-reviewed before sale, and quality varies.
- CreatineCreatine is a compound your liver and kidneys make from amino acids, and it is also in meat and fish. Muscles use it to help make ATP for short bursts of energy. Creatine monohydrate is the most studied form. It often causes weight gain from water. People with kidney disease should ask a doctor first.
- ElderberryElderberry is the dark purple berry of the European or black elder tree (Sambucus nigra), promoted as a supplement for colds and flu. NIH's NCCIH says only a small number of studies exist. Raw or unripe berries, leaves and stems contain cyanide-producing substances; cooking removes the toxin. Little is known about safety in pregnancy.
- Glucosamine and ChondroitinGlucosamine and chondroitin are natural parts of joint cartilage, sold in the US as dietary supplements, alone or together. Labels show glucosamine sulfate or hydrochloride and chondroitin sulfate. Research for knee osteoarthritis is inconsistent, and guidelines disagree. Glucosamine may raise blood sugar in some people, and both are linked to bleeding with warfarin.
- L-TheanineL-theanine is an amino acid found mainly in tea leaves; a 200 ml cup of tea has 6 to 32 mg. It is sold as a supplement, often with caffeine. Studies on relaxation, attention and sleep are small and inconsistent. No US or UK agency sets a daily amount, and safety data are mostly from animal studies.
- MelatoninMelatonin is a hormone your brain makes in response to darkness; it helps time your body clock and sleep. The US sells it as a dietary supplement, but the UK makes it prescription-only. Short-term use appears safe for most people; long-term safety is unclear. Ask a health care provider before giving it to children.
- Omega-3 (Fish Oil)Omega-3s are fats the body uses to build cell membranes. ALA comes from plant oils; EPA and DHA come from fish, fish oil, krill oil and algal oil. A typical 1,000 mg fish oil capsule has about 180 mg EPA and 120 mg DHA. Ask your pharmacist first if you take warfarin.
- ProbioticsProbiotics are live microorganisms, mostly Lactobacillus and Bifidobacterium bacteria plus some yeasts. Their effects depend on the exact strain, and a higher CFU count is not necessarily better. Some yogurts contain them. People who are seriously ill or immunocompromised, and premature infants, have a higher risk of serious infection.
- Psyllium FiberPsyllium is a bulk-forming fiber from plantago seed that absorbs liquid in the intestines and swells. Take each dose with at least 8 ounces (240 ml) of water or other fluid, because too little liquid can make it swell and cause choking. The NHS advises taking other medicines more than an hour before or after it.
- Turmeric and CurcuminTurmeric is a spice from a plant in the ginger family; curcumin is a main compound that gives it its yellow color. Many supplements add piperine from black pepper or use special forms to raise absorption. Liver damage has been reported with some of these high-absorption products. Turmeric supplements may be unsafe in pregnancy.
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Educational information, not medical advice. Check with your doctor or pharmacist before starting or changing supplements.