B12 helps your body make red blood cells, so running low can leave you weak and short of breath. If you are tired and in a group that often runs low, have your B12 checked before you buy. If your level is normal, studies have not found that extra B12 adds energy.
If you are low, see what to take.
Tired and at risk? Get a B12 blood test first
Low B12 is more likely if one of these fits you:
- A vegan diet, or little animal food and no fortified foods
- Metformin, or an acid reducer such as omeprazole or famotidine
- Stomach or weight-loss surgery
- An autoimmune condition, such as thyroid disease or type 1 diabetes
- Colchicine, or some seizure and nerve-pain medicines
- Recreational nitrous oxide (“laughing gas”)
These are the common risk factors doctors use to decide who to test (NICE 2024). Age matters too. After 50, up to 3 in 10 people absorb B12 from food poorly (IOM 1998). More in B12 for seniors.
Besides tiredness, watch for brain fog or memory trouble, pins and needles, balance problems, a sore and smooth tongue, or anemia on a blood count (NICE 2024). Nerve signs can show up before anemia does (NIH). See all the signs of low B12.
Ask for a B12 level, not just a blood count. A borderline result can be checked with an MMA test. If you use nitrous oxide, say so, because a different test is used. Finding a shortage early matters, since it can leave lasting nerve damage (NIH). If your level is normal, ask about other causes of tiredness, such as low iron.
If your test shows you are low
Many people bring a low level back up with a 1,000 mcg pill a day, the most common size: about 1 in 3 of the 203 B12 labels we read hold exactly that. Your doctor sets the dose, and may use shots for some causes.
NatureWise Vitamin B12 1000 mcg
Our pick. $0.12 a day, on NSF’s testing list. The softgel has gelatin, so it is not vegan.
If your doctor prefers methylcobalamin, Thorne Vitamin B12 (methylcobalamin) is on two NSF testing lists. For a vegan chewable, try Jarrow Methyl B-12 1000 mcg.
If your doctor wants a higher dose, NOW Methyl B-12 5000 mcg or Solgar Methylcobalamin 5000 mcg hold 5,000 mcg each.
Frequently asked questions
Do B12 shots work better for energy than pills?
Shots and pills both help by raising a low B12 level. In small trials, high-dose pills raised it about as well as shots, but the trials did not measure how people felt (Wang 2018). Doctors use shots when the gut cannot absorb B12, such as with pernicious anemia.
Can metformin make me tired because of B12?
It can, over years. Metformin can lower B12 over time (de Jager 2010). If you take metformin and feel tired or have tingling, ask your doctor to check your B12. Do not stop metformin on your own.
How long does B12 take to help tiredness?
In people who were low, blood levels rose within 4 weeks on 500 mcg a day (Sharabi 2003). Feeling better can take longer. Once your doctor says your level is normal, a 100 mcg tablet like Carlyle Vitamin B12 100 mcg can keep it there. If you are still tired at that point, tell your doctor. More in how long B12 takes to work.
Is 1,000 mcg of B12 a day too much?
No upper limit has been set for B12 (NIH). Read is 1,000 mcg of B12 too much? for who should check with a doctor first.
Related guides
- B12 deficiency signs: every symptom and the two blood tests
- B12 dosage guide: how much to take, by age and situation
- Best B12 supplements: our full ranking
Sources
- NIH Office of Dietary Supplements. "Vitamin B12: Fact Sheet for Health Professionals" (energy, deficiency signs, nerve damage, NHANES prevalence, no upper limit). ods.od.nih.gov (updated July 2, 2025; read October 15, 2026).
- NICE. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239), 2024 (common signs, risk factors, when to test; nitrous oxide testing). Read October 16, 2026.
- Institute of Medicine. Dietary Reference Intakes, chapter 9: Vitamin B12. 1998 (adults over 50).
- Markun S, et al. "Effects of Vitamin B12 Supplementation on Cognitive Function, Depressive Symptoms, and Fatigue: A Systematic Review, Meta-Analysis, and Meta-Regression." Nutrients. 2021. PMID: 33809274
- Wang H, et al. "Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency." Cochrane Database Syst Rev. 2018. PMID: 29543316
- de Jager J, et al. "Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency." BMJ. 2010. PMID: 20488910
- Sharabi A, et al. "Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route." Br J Clin Pharmacol. 2003. PMID: 14616423
- Prices are each brand’s list price, checked October 15, 2026. Amazon’s price may differ.