Most supplements aren’t worth your money. The supplement industry generates tens of billions of dollars annually, but only a handful of products have consistent evidence behind them. The U.S. Preventive Services Task Force has found insufficient evidence to recommend multivitamins for preventing heart disease or cancer in healthy adults. That doesn’t mean all supplements are useless, though. A few fill genuine nutritional gaps or provide benefits that are hard to get from food alone.
Vitamin D
Vitamin D is the supplement with the strongest case for widespread use. Your body produces it from sunlight, but if you live above roughly the 37th parallel (think San Francisco, St. Louis, or Richmond), you can’t make enough from sun exposure for several months of the year. People with darker skin, those who work indoors, and anyone who regularly wears sunscreen are also at higher risk of running low.
The National Academies of Sciences considers blood levels of 50 nmol/L (20 ng/mL) or higher adequate for bone and overall health. Below 30 nmol/L (12 ng/mL), you’re in deficiency territory, which over time can soften bones and weaken muscles. The range between 12 and 20 ng/mL is considered inadequate for most people. Levels above 50 ng/mL, on the other hand, are linked to potential harm, so more is not better.
For most adults, 1,000 to 2,000 IU daily is enough to maintain healthy levels. Vitamin D is fat-soluble, meaning your body absorbs it alongside dietary fat in the small intestine. Taking it with a meal that contains some fat, even just a handful of nuts or eggs, meaningfully improves absorption. If you’ve never had your levels checked, a simple blood test can tell you where you stand.
Magnesium
Magnesium is involved in over 300 enzyme reactions in your body, including muscle contraction, nerve signaling, and blood sugar regulation. Many people don’t get enough from food, partly because modern soil depletion has reduced magnesium content in crops and partly because processed foods are poor sources.
The tricky part is choosing the right form. Magnesium oxide, the cheapest option on most shelves, is poorly absorbed and mostly useful as a laxative. Better-absorbed forms include magnesium citrate (highly bioavailable, though it can loosen stools at higher doses), magnesium glycinate (well absorbed and gentle on the stomach), and magnesium malate (also well absorbed, with less laxative effect than citrate). Magnesium L-threonate is a newer form that animal research suggests is particularly effective at reaching brain cells, making it popular for cognitive support, though human data is still limited.
If you’re choosing a general-purpose magnesium supplement, glycinate or malate are solid picks. Most people benefit from 200 to 400 mg per day. Signs you might be low include muscle cramps, poor sleep, and feeling wired but tired.
Omega-3 Fatty Acids
If you eat fatty fish like salmon, mackerel, or sardines two or more times per week, you probably don’t need an omega-3 supplement. If you don’t, you likely aren’t getting enough EPA and DHA, the two omega-3 fats your body actually uses. Plant-based omega-3 from flaxseed or walnuts is a different compound (ALA) that your body converts to EPA and DHA very inefficiently, at rates typically below 10%.
Omega-3s play a structural role in cell membranes throughout the body, particularly in the brain and retina. They also help regulate inflammation. For general health, most guidelines suggest around 250 to 500 mg of combined EPA and DHA per day. Higher doses, in the range of 2 to 4 grams daily, have been studied for cardiovascular benefits, but those should be discussed with a doctor since high-dose fish oil can thin the blood.
Quality matters here more than with most supplements. Fish oil can oxidize (go rancid), which may negate its benefits. Store capsules in the fridge, and if they smell strongly fishy when you break one open, they’ve likely degraded. Algae-based omega-3 supplements are a good alternative for vegetarians and vegans, providing DHA and sometimes EPA directly.
Creatine
Creatine is one of the most studied supplements in existence, and it’s not just for bodybuilders. Your body makes creatine naturally and stores it mostly in muscle, where it helps regenerate the energy molecule your cells run on. Supplementing with creatine monohydrate increases those stores, which improves strength, power output, and recovery from exercise.
What’s less widely known is that creatine also accumulates in the brain. A meta-analysis published in Frontiers in Nutrition found that creatine supplementation significantly improved memory and reduced the time needed to complete attention and processing speed tasks. The benefits appeared strongest in women, adults aged 18 to 60, and people with existing health conditions. It did not show significant effects on executive function or overall cognitive performance in the pooled data.
The standard dose is 3 to 5 grams of creatine monohydrate per day. You don’t need a loading phase, and you don’t need fancy forms like creatine hydrochloride or buffered creatine. Plain monohydrate is the most researched, the most effective, and the cheapest. It dissolves well in water and has an excellent long-term safety profile.
Vitamin B12
B12 is essential for nerve function, red blood cell production, and DNA synthesis. It’s found almost exclusively in animal products: meat, fish, eggs, and dairy. If you follow a vegan or strict vegetarian diet, supplementing B12 isn’t optional. Deficiency causes fatigue, numbness and tingling in the hands and feet, difficulty thinking clearly, and, if prolonged, irreversible nerve damage.
Even some meat-eaters run low, particularly adults over 50, whose stomachs produce less of the acid needed to extract B12 from food. People taking acid-reducing medications are also at elevated risk. The recommended daily intake varies by country: the UK advises 1.5 mcg, the US advises 2.4 mcg, while some researchers argue that up to 10 mcg in divided doses is closer to what the body actually needs. Because B12 is water-soluble and excess is excreted in urine, there’s very little risk of taking too much.
What Probably Isn’t Worth It
Multivitamins are the most popular supplement in the country, yet the evidence behind them is surprisingly thin. The USPSTF reviewed the available research and issued an “I statement,” meaning there isn’t enough evidence to say whether multivitamins help or hurt when it comes to preventing chronic disease in healthy, non-pregnant adults. You’re better off identifying specific gaps in your diet and filling those directly.
Vitamin C supplements are unnecessary for most people. A single orange or bell pepper gives you more than a day’s worth, and megadoses don’t prevent colds (though they may shorten them by a few hours). Biotin supplements are heavily marketed for hair and nails, but deficiency is extremely rare in people eating a normal diet, and no good evidence supports megadosing for cosmetic benefits. Collagen supplements have some emerging data for skin elasticity, but the research is still mixed and largely industry-funded.
Avoiding Supplements That Can Harm You
Fat-soluble vitamins (A, D, E, and K) accumulate in body tissue rather than being flushed out in urine, which means they carry real toxicity risk at high doses. The European Food Safety Authority sets the tolerable upper limit for preformed vitamin A at 3,000 mcg per day for adults. For zinc, the upper limit is 25 mg daily. Iron supplements should only be taken if you have a confirmed deficiency, because excess iron builds up in organs. The safe upper level for adults is 40 mg per day, and symptoms of iron overload include nausea, joint pain, and liver damage over time.
The supplement industry in the US is largely self-regulated. Products don’t need to prove they work before hitting shelves, and independent testing routinely finds supplements that contain less of the active ingredient than listed, or are contaminated with heavy metals or unlisted compounds. Look for products certified by third-party testing organizations like NSF International or USP. These certifications verify that what’s on the label is actually in the bottle and that the product is free from harmful contaminants.
Getting the Most From Your Supplements
Timing and context affect how well your body absorbs what you take. Vitamins A, D, E, and K all require dietary fat for proper absorption. Taking them on an empty stomach means a significant portion passes through you unused. A meal with even a modest amount of fat is enough.
Magnesium and zinc compete for absorption, so if you take both, separate them by a few hours. Iron absorption is enhanced by vitamin C and inhibited by calcium, coffee, and tea. Taking iron on an empty stomach improves absorption but can cause nausea, so many people take it with a small amount of food as a compromise.
Creatine, B12, and omega-3s don’t have strong timing requirements. Consistency matters more than timing for all of these. A supplement you take reliably every day at whatever time works for your routine will always outperform one you take sporadically at the “optimal” time.