The supplement industry is built on an asymmetry: the cost of making a claim is near zero, and the cost of disproving one is a multi-year clinical trial. That leaves buyers sorting a shelf where a substance with hundreds of randomized controlled trials sits next to one with a press release, at similar prices, in similar packaging.
This hub sorts them. Every guide here is evidence-ranked, dose-specific, and explicit about which supplements only work if you are actually deficient — because that last category is where most supplement spending is wasted.
The Verdict
The evidence tiers
Ranked by strength of the underlying human evidence, not by popularity or marketing spend. "Strong" means multiple randomized controlled trials with consistent, replicated results at the stated dose. "Moderate" means real trial support with smaller effect sizes or fewer replications. "Weak" means the mechanism is plausible but human outcomes have not delivered.
| Supplement | Evidence | Effective dose | What the data actually shows |
|---|---|---|---|
| Creatine monohydrate | Strong | 5g/day | Hundreds of RCTs. Muscle, strength, and cognitive benefits. The most-validated supplement in existence. |
| Vitamin D3 | Strong (if deficient) | 1,000–2,000 IU/day | Corrects a genuinely common deficiency. Useless if you are already replete — test first. |
| Omega-3 (EPA/DHA) | Strong | 1.5–3g/day combined | Cardiovascular, inflammatory, and cognitive support. Dose to an omega-3 index above 8%. |
| Magnesium glycinate | Moderate–strong | 200–400mg elemental | Sleep quality, anxiety, and repletion of a widely-underconsumed mineral. |
| Protein powder (whey or blend) | Strong | To hit daily protein target | Not exotic — just a convenient way to reach 0.7–1g per lb lean mass. |
| Ashwagandha (KSM-66/Sensoril) | Moderate | 300–600mg/day | Multiple RCTs on cortisol reduction in stressed adults. Effects real but modest. |
| Iron (bisglycinate) | Strong (if deficient) | Only if ferritin is low | One of the most-missed causes of fatigue, especially in menstruating women. |
| B12 (methylcobalamin) | Strong (if deficient) | 500–1,000mcg/day | Deficiency causes real fatigue and cognitive symptoms; ~10% of adults over 50 are low. |
| Berberine | Moderate | 500mg 2–3×/day | Meaningful glucose and lipid effects. GI side effects common. |
| Collagen peptides | Moderate | 10–15g/day | Skin elasticity and joint discomfort have replicated trial support. Other claims do not. |
| NMN / NR (NAD+ precursors) | Weak–emerging | 250–500mg/day | Raises NAD+ biomarkers. Direct healthspan outcomes remain unproven in humans. |
| Resveratrol | Weak | — | Poor bioavailability. Human results have not replicated the early animal findings. |
Test before you supplement — the six that require it
For most supplements, taking them without deficiency produces no benefit. For a few, it produces active harm. These six are where a blood test before purchase changes the decision.
| Supplement | What to test | Why it matters |
|---|---|---|
| Vitamin D | 25-hydroxyvitamin D | Optimal 50–70 ng/mL — supplementing blind can overshoot |
| Iron | Ferritin + CBC | Supplementing iron without deficiency is actively harmful |
| B12 | Serum B12 (+ MMA if borderline) | Excess is excreted, but treating the wrong cause wastes months |
| Omega-3 | Omega-3 index (RBC) | Dose to target; absorption varies 2–3× between individuals |
| Magnesium | RBC magnesium (serum is unreliable) | Serum magnesium stays normal until deficiency is severe |
| Cortisol-targeted supplements | 4-point salivary cortisol | Confirms the pattern before you treat it |
How to read a supplement label
Four things separate a legitimate product from an expensive placebo, and all four are visible on the label before you buy.
- Elemental content, not compound weight. A capsule listing "400mg magnesium glycinate" contains roughly 50–60mg of actual elemental magnesium. Reputable brands print both. Brands that print only the salt weight are relying on the bigger number looking better.
- The form matters more than the brand. Magnesium oxide is poorly absorbed and functions mostly as a laxative; glycinate and threonate are absorbed well. Folic acid is inferior to methylfolate for the large share of people carrying MTHFR variants. The form is a bigger quality signal than the label design.
- Third-party testing. NSF Certified for Sport, Informed Sport, and USP verification each mean an outside lab confirmed the contents. In an industry regulated as food rather than medicine, this is the only real assurance that the capsule contains what the label claims.
- No proprietary blends. A "proprietary blend" discloses total weight but hides the per-ingredient dose. This exists almost exclusively so the manufacturer can include a study-backed ingredient at a fraction of the study dose. If the per-ingredient amounts are not printed, assume they are too low to matter.
The five mistakes that waste the most money
- Supplementing without a baseline. Without a starting measurement there is no way to know whether a supplement corrected anything. This is the single largest source of wasted supplement spend.
- Stacking 15+ items. Beyond about five supplements, interactions and redundancy accumulate and attribution becomes impossible. If everything changes at once, nothing is measurable.
- Under-dosing. Most consumer multivitamins and blends contain clinically meaningless amounts of their headline ingredients. Ashwagandha below 300mg, omega-3 below 1g combined EPA/DHA, and creatine below 3g are all effectively placebos.
- Chasing novelty. The supplement with the newest podcast coverage is, by definition, the one with the least human evidence. Creatine has been studied since the 1990s — that is a feature.
- Treating supplements as a substitute for the fundamentals. No stack compensates for chronic short sleep, no aerobic training, or excess alcohol. Supplements operate at the margin of a foundation that has to already exist.
Buying guides
- Best cortisol supplement — ashwagandha, phosphatidylserine, rhodiola and what the evidence supports
- Best magnesium supplement — glycinate vs threonate vs citrate vs oxide
- Best time to take vitamin D — why fat matters more than time of day
- Best vitamins for energy — what works when you are deficient, and what does not
- Creatine and kidney markers — why it raises creatinine and lowers eGFR without harming the kidney
- Creatine for longevity — the best-evidenced option here, and the lab artifact it creates
- Creatine for women — dosing, the menopause and bone evidence, and the water-weight question
- NMN vs NR — the NAD+ precursors, their trials, and the 2025 regulatory reversal
- Magnesium side effects: the dose-dependent gut effects, the 350 mg limit, and the kidney exception
- NMN dosage: what each human trial administered, against what the labels recommend
- NMN side effects: what the trials reported, and how far that reassurance extends
- Senolytic supplements — fisetin, quercetin and what the human trials actually showed
Related
- Biomarker guides — the tests that tell you what to supplement
- Normal vs optimal ranges
- Nutrient density — food-first alternatives to supplementation
- Nutrition guides
Frequently Asked Questions
What supplements are actually worth taking?
Creatine monohydrate has the strongest evidence base of any supplement. Vitamin D3, omega-3, iron, and B12 are worth taking if you are deficient — test first. Magnesium glycinate and protein powder are broadly useful. Ashwagandha and berberine have moderate, real evidence for specific outcomes. Most of the rest of the supplement aisle is marketing.
Should I test before supplementing?
For vitamin D, iron, B12, omega-3, and magnesium — yes. These are the supplements where the benefit depends entirely on whether you are deficient, and where over-supplementing carries real risk (iron especially). For creatine, protein, and magnesium glycinate at standard doses, testing is optional.
How many supplements should I take?
Fewer than most people take. A defensible stack for a healthy adult is often 3–5 items: creatine, vitamin D (if low), omega-3, magnesium, and protein powder. Stacks of 15+ supplements almost always contain redundant, sub-therapeutic, or unsupported items — and make it impossible to know what is working.
Are expensive supplements better?
Rarely. Price correlates more with marketing spend than with quality. What matters is: the right form (magnesium glycinate vs oxide), a clinically-relevant dose, third-party purity testing, and the elemental content printed on the label. A $15 third-party-tested creatine outperforms a $60 proprietary blend.
What is the biggest mistake people make with supplements?
Supplementing without measuring, then never re-measuring. The second biggest is buying "proprietary blends" that hide per-ingredient doses — almost always because the doses are below what the underlying studies used. The third is treating supplements as a substitute for sleep, training, and diet.
Do supplements interact with medications?
Yes, meaningfully. St. John's wort affects many drug metabolisms. Vitamin K interacts with warfarin. Berberine can compound with metformin and glucose-lowering drugs. High-dose fish oil interacts with anticoagulants. Anyone on prescription medication should review a supplement stack with a physician or pharmacist.