What the evidence shows
- Caffeine has the clearest evidence for short-term alertness in healthy adults.
- Iron may reduce self-reported fatigue in iron-deficient adults, but trials did not show better exercise capacity.
- Extra vitamin B12 did not reduce fatigue in one trial of people with normal B12 levels.
- Ginseng trial results are mixed, and reported benefits remain uncertain.
- These studies do not establish that any supplement boosts everyday energy for everyone.
What “energy” means in these studies
People use “energy” to mean alertness, less tiredness, or better exercise. These are different outcomes, so one result cannot stand for all three. Caffeine has the clearest trial evidence for short-term alertness. Iron has evidence for less fatigue in people with low iron stores. Extra vitamin B12 has not reduced fatigue in one trial of people with normal B12 levels. Ginseng results vary across reviews. None of these findings proves that a supplement raises everyday energy for everyone.
Caffeine: strongest evidence for short-term alertness
A 2025 review pooled 31 randomized, double-blind, placebo-controlled trials. They tested pure caffeine in 1,455 rested, healthy adults. Caffeine improved attention accuracy and reaction time, with small average effects. The review measured attention, not lasting energy or relief from chronic fatigue. It also found that higher amounts did not keep improving accuracy. This evidence supports a short-term alertness effect in the studied adults. Read the review.
Iron: evidence applies to low iron stores
Iron is the clearest example of a supplement that may help a specific group. A review of randomized trials in iron-deficient adults without anemia found less self-reported fatigue. It found no clear improvement in objective exercise capacity. The review included 18 trials and 1,170 people; only four trials measured fatigue. One trial subgroup involved menstruating women with low or borderline iron stores. These results do not show that iron boosts energy when iron levels are adequate. Read the review.
Vitamin B12: extra amounts did not ease fatigue
Vitamin B12 deficiency can cause fatigue, so correcting a deficiency addresses a real lack. But extra B12 has not shown a general energy effect. In a randomized, double-blind trial, 95 adults with irritable bowel or inflammatory bowel disease had fatigue and normal B12 levels. After eight weeks, B12 did not reduce their fatigue more than placebo. This trial does not answer whether correcting a confirmed deficiency improves symptoms. Read the trial and MedlinePlus on B12 deficiency.
Ginseng: mixed findings and limited certainty
Ginseng trials have studied people with chronic or disease-related fatigue, as well as people reporting daily fatigue. A 2023 review pooled 19 randomized trials and found no clear overall reduction in fatigue. Some smaller subgroups showed benefit, including chronic fatigue and ginseng herbal formulas. The review described those effects as small and called for more rigorous trials. Different ginseng products and study groups make the results hard to compare. Read the review.
How to read the evidence
Randomized, placebo-controlled trials give stronger evidence than personal reports. Reviews can combine trials, but their conclusions still depend on the studies they include. Caffeine trials mainly measured brief attention changes in healthy adults. Iron trials focused on people with low iron stores, and fatigue reports did not match better exercise capacity. B12 evidence here comes from one trial in people with normal levels. Ginseng results differ across groups and products. The best-supported effects are specific, not a general promise of more daily energy.
Sources
- A systematic review and meta-analysis of the acute effect of caffeine on attention
- Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials
- Surplus vitamin B12 use does not reduce fatigue in patients with Irritable Bowel Syndrome or inflammatory bowel disease
- Ginseng and Ginseng Herbal Formulas for Symptomatic Management of Fatigue: A Systematic Review and Meta-Analysis
- Vitamin B12 deficiency anemia: MedlinePlus Medical Encyclopedia
About this article
NootroWorld publishes research summaries, not medical advice. This article describes what studies have and have not established; it does not tell you what to take. Supplements can interact with medication, pregnancy and existing health conditions, and study populations often differ from your own situation. Discuss any personal decision with a qualified healthcare professional.