Research, evidence, and sources
What this page is for
An afternoon slump is not, by itself, a vitamin deficiency. Only a clinician can decide whether a symptom needs a test.
What the reviewed sources say
“Manufacturers often promote vitamin B12 supplements for energy, athletic performance, and endurance. However, vitamin B12 doesn’t provide these benefits in people who get enough B12 from their diet.”
Vitamin B12 helps keep blood and nerve cells healthy and helps make DNA. It also helps prevent megaloblastic anemia, a blood condition that makes people tired and weak. If someone has a vitamin B12 deficiency, they may feel tired or weak. Those symptoms can take years to appear because the body stores a large amount.
Manufacturers often promote vitamin B12 supplements for energy, athletic performance, and endurance. The fact sheet says vitamin B12 does not provide those benefits in people who already get enough from their diet. Research has not shown that any supplemental form — cyanocobalamin, methylcobalamin, or the others named on the page — is better than the others.
The adult recommended amount on the fact sheet is 2.4 micrograms a day. Some supplements contain 500 or 1,000 micrograms. The body absorbs only a small percentage of those large doses, and the fact sheet says the doses are considered safe. That is label context, not a dose for you. People over 50, people who eat little or no animal food, and people with certain stomach conditions are among those who may not get enough from diet alone. Most people in the United States do get enough from food.
Vitamin B12 — Consumer Fact Sheet NIH Office of Dietary Supplements. Updated 2023-12-15. Retrieved 2026-10-07.
“Symptoms of iron deficiency anemia include GI upset, weakness, tiredness, lack of energy, and problems with concentration and memory.”
Iron is a different question. The link below opens iron-only labels. It is not an energy recommendation. The body uses iron to make hemoglobin, which carries oxygen, and myoglobin, which provides oxygen to muscles. Iron deficiency anemia can include weakness, tiredness, and lack of energy, and it can make exercise harder. Most people in the United States get enough iron. Extra iron can upset the stomach, and very high doses are dangerous. Accidental overdose of iron products is a leading cause of fatal poisoning in children under 6.
Iron — Consumer Fact Sheet NIH Office of Dietary Supplements. Updated 2023-08-17. Retrieved 2026-10-07.
How to compare a label
“The nutrition label for a dietary supplement is called a “Supplement Facts” panel.”
Start with the Supplement Facts panel. The FDA’s labeling guide uses that name for a dietary supplement’s nutrition label.
Read the ingredient name, the amount per serving, and the serving size. A blend that lists several ingredients is a different product from a bottle that is only one of them. The price on a retailer card is the package price, not a price per serving, and a sale ribbon is not a checked comparison price.
- Dosage unit converterConvert grams, milligrams, and micrograms. IU converts only when the vitamin form is known.
- Price per servingSee what one serving costs from the container price and the servings on the label.
- Cost per labeled amountSee what a labeled amount of an ingredient costs. A second product is optional.
- Daily nutrient targetsPublished folate, magnesium, and vitamin A figures for a life stage.
Dietary Supplement Labeling Guide: Chapter IV. Nutrition Labeling U.S. Food and Drug Administration. Retrieved 2026-10-05.
Labels you can open
These lists match a published ingredient. Opening one is not a suggestion that the ingredient meets this goal.
Sources
- Vitamin B12 — Consumer Fact Sheet NIH Office of Dietary Supplements. Updated 2023-12-15. Retrieved 2026-10-07.
- Iron — Consumer Fact Sheet NIH Office of Dietary Supplements. Updated 2023-08-17. Retrieved 2026-10-07.
- Dietary Supplement Labeling Guide: Chapter IV. Nutrition Labeling U.S. Food and Drug Administration. Retrieved 2026-10-05.
