Stand in any pharmacy's supplement aisle and the implied message is "all of them." The evidence says something considerably less profitable for that aisle: for a generally healthy adult eating a reasonable diet, most supplements do nothing measurable. Large, long-running studies of routine multivitamin use in healthy populations consistently find little to no effect on chronic disease outcomes.
But that's only half the picture. Specific people have specific, real gaps — and for them, the right supplement genuinely matters. The useful question isn't "should I take supplements" in the abstract, but whether you specifically fall into a group with an identifiable gap. That comes down to four questions.
Question 1: What does your diet leave out?
This is the single most reliable predictor available. Deficiencies follow exclusions in a fairly mechanical way: a vegan or vegetarian diet creates near-certain B12 risk (it exists naturally only in animal foods), with iron and omega-3 also worth watching. Little or no dairy intake means calcium and vitamin D need another route. Rarely eating fish means likely-inadequate omega-3 intake. A diet heavy in ultra-processed food and light on plants commonly runs short on magnesium and fiber.
Question 2: What life stage are you in?
Anyone who could become pregnant benefits from folic acid (400mcg daily) — it's arguably the single best-evidenced preventive supplement that exists, protecting against neural tube defects in the earliest weeks of pregnancy, often before pregnancy is even confirmed. Adults over 65 face two quiet physiological shifts: skin produces less vitamin D from the same sun exposure, and the stomach releases less B12 from food — both worth checking even without symptoms at this age.
Question 3: What does your lifestyle limit?
Spending most days indoors, living at northern latitudes, using sunscreen consistently (as recommended), or having darker skin each independently raise vitamin D risk — and in combination, particularly during winter months, deficiency becomes more likely than not. Heavy endurance training accelerates iron turnover. Certain common medications — metformin and long-term acid reducers for B12, diuretics for magnesium — deplete specific nutrients in ways patients are rarely told about at the time of prescription.
Question 4: What are your symptoms actually telling you?
Symptoms are the noisiest signal of the four, since nearly everything overlaps with ordinary stress and poor sleep — but they're far from useless in combination with the other three questions. Unexplained fatigue has three usual nutritional suspects: iron, B12, and vitamin D, and the accompanying details usually help distinguish them — pale skin and breathlessness lean iron, tingling and brain fog lean B12, low winter mood and muscle aches lean vitamin D.
What you probably don't need
A few honest subtractions worth naming, since no one selling supplements tends to volunteer them: a "just in case" multivitamin is reasonable as cheap insurance but won't correct a genuine deficiency (the doses are typically too low); megadoses of anything accumulate risk, particularly the fat-soluble vitamins A, D, E, and K; iron without a confirmed blood test is genuinely risky given how the body handles excess iron; and any supplement you can't clearly name the specific gap for is likely decoration rather than function.
Our free assessment walks through all four questions systematically in about two minutes — diet, life stage, lifestyle, and symptoms together — rather than leaving you to guess which trend to follow.
Why a blood test isn't always the obvious first step
A reasonable question is why not simply test everything and skip the guesswork. In practice, comprehensive nutrient panels are expensive, not always covered by insurance, and — as covered elsewhere on this site — some deficiencies (magnesium being the clearest example) don't reliably show up on standard blood tests even when a real tissue-level shortfall exists. The more efficient approach used by most clinicians, and the one our own assessment is modeled on, is using the four questions above to narrow down which specific nutrients are actually worth testing for, rather than testing broadly and hoping something turns up.
Framework informed by NIH Office of Dietary Supplements guidance across individual nutrient fact sheets.