The Most Important Thing to Know
Persistent fatigue is a symptom, not a diagnosis. Before supplementing for energy, the most important step is identifying why you're fatigued. Common treatable causes include iron deficiency (even without anemia), thyroid dysfunction, sleep disorders, depression, vitamin D deficiency, and chronic dehydration. A basic blood panel can identify most of these. Supplements only help when they address the actual cause.
Evidence Summary
| Supplement | Evidence | Key Finding | Only Helps If... |
|---|---|---|---|
| Iron | Strong | d=0.34–1.01 for fatigue reduction[1][2] | Iron deficient (test ferritin) |
| Creatine | Moderate | SMD=0.29 for memory; 0.88 in elderly[3] | Cognitive fatigue, age-related decline |
| Vitamin B12 | Weak | No significant effect in non-deficient (p=0.71)[5] | B12 deficient (vegans, elderly, pernicious anemia) |
| CoQ10 | Weak | Not significant for fatigue (SMD -0.33, p=0.54)[7] | On statins (which deplete CoQ10) |
Iron — The Strongest Evidence
Iron has the most clinically actionable evidence for fatigue of any supplement. A 2025 meta-analysis of 18 studies (1,408 participants) found iron supplementation improved fatigue with effect sizes of d=0.34 in RCTs and d=1.01 in pre-post studies — even in non-anemic iron-deficient individuals.[2] It also improved cognitive intelligence (d=0.46) and short-term memory (d=0.53).
A separate network meta-analysis of 34 RCTs (4,688 participants) found ferric carboxymaltose was the most effective form, significantly increasing hemoglobin and improving quality of life for fatigue.[1]
Critical caveat: Iron supplementation should never be started without blood testing confirming deficiency. Excess iron is toxic and can damage the liver, heart, and pancreas. Test serum ferritin — not just hemoglobin — because iron deficiency causes fatigue well before anemia develops.
Creatine — Cognitive Energy
Creatine is primarily known for gym performance, but its role in brain energy metabolism makes it relevant for cognitive fatigue. A meta-analysis of 10 RCTs found creatine improved memory vs. placebo (SMD=0.29, p=0.02), with a dramatically larger effect in older adults aged 66–76 (SMD=0.88) compared to younger adults (SMD=0.03).[3]
Creatine maintains brain ATP levels during cognitive demand — essentially helping the brain sustain energy during tasks that would normally cause mental fatigue. A separate study found a single high dose (0.35 g/kg) partially reversed cognitive decline from 21 hours of sleep deprivation.[4]
This makes creatine particularly interesting for older adults experiencing cognitive fatigue and for anyone dealing with sleep deprivation — but it does not increase subjective "energy" or reduce physical tiredness.
Vitamin B12 — Only if Deficient
B12 is perhaps the most over-supplemented vitamin for energy. A meta-analysis of 18 articles (2,015 participants) found no significant effect of B vitamin supplementation on fatigue in non-deficient populations.[5] Separate meta-analyses in fibromyalgia and MS patients also found no meaningful B12 differences vs. healthy controls.[6]
B12 deficiency does cause fatigue, brain fog, and neurological symptoms — but it's only common in: vegans/vegetarians, adults over 60 (reduced absorption), people taking metformin or PPIs, and those with pernicious anemia. If you eat animal products and are under 60, B12 deficiency is unlikely, and supplementation won't help your energy.
CoQ10 — Disappointing Evidence
A 2026 meta-analysis of 5 RCTs (474 participants) found no significant benefit of CoQ10 for fatigue (SMD: -0.33, 95% CI: -1.38 to 0.72, p=0.54) with very high heterogeneity (I²=89%).[7] A separate meta-analysis of 24 RCTs found CoQ10 showed "at most modest and context-dependent benefits for exercise performance" that disappeared after excluding outlier studies.[8]
The main evidence-based use case for CoQ10 is in people taking statins, which can deplete CoQ10 levels and cause muscle fatigue. In that specific population, supplementation may help. For general fatigue without statin use, the evidence doesn't support it.
What About Ashwagandha, Rhodiola, Ginseng?
Adaptogens are marketed heavily for energy, but their evidence for fatigue is primarily indirect — through stress/cortisol reduction rather than direct energy enhancement. Ashwagandha has strong evidence for anxiety and stress, which may secondarily improve perceived energy. But this is a different mechanism than correcting a nutrient deficiency.
What to Consider
- Get bloodwork first. Ferritin, B12, vitamin D, thyroid panel (TSH, T3, T4), CBC. This catches the most common correctable causes of fatigue.
- Sleep quality matters more than supplements. No supplement compensates for chronic sleep deprivation or undiagnosed sleep apnea.
- Iron is the only supplement with strong fatigue evidence — and only when you're deficient. Never supplement iron without testing.
- Creatine for brain energy is an underappreciated finding, especially for older adults (SMD=0.88 for memory). At 3–5 g/day with an excellent safety profile, it's worth considering for cognitive fatigue.
References
- Xue Q, et al. "IV iron for perioperative iron deficiency anemia: network meta-analysis." J Clin Anesth. 2026. 34 RCTs, 4,688 participants. PubMed
- Fiani D, et al. "Iron supplementation: psychiatric and cognitive outcomes in non-anemic populations." Neurosci Biobehav Rev. 2025. 18 studies, 1,408 participants. PubMed
- Prokopidis K, et al. "Creatine supplementation on memory: meta-analysis of RCTs." Nutr Rev. 2023. 10 RCTs. PubMed
- Gordji-Nejad A, et al. "Single dose creatine and cognitive performance during sleep deprivation." Sci Rep. 2024. PubMed
- Young LM, et al. "B vitamin supplementation on mood." Nutrients. 2019. 18 articles, 2,015 participants. PubMed
- Ismail O, et al. "Ferritin, vitamin D, and B12 in fibromyalgia: meta-analysis." Musculoskelet Care. 2025. 25 studies. PubMed
- Magalhaes PLM, et al. "CoQ10 on depression and fatigue: meta-analysis of RCTs." J Clin Psychopharmacol. 2026. 5 RCTs, 474 participants. PubMed
- Deng H, et al. "CoQ10 and exercise performance: meta-analysis." Br J Nutr. 2025. 24 RCTs. PubMed