Free tool · GLP-1 nutrition
Every B vitamin, compared for life on a GLP-1
Eating less on a GLP-1 also means less food carrying vitamins in. Pick any of the eight B vitamins to see how much adults need, where the upper limit sits, which foods supply it, how the supplement forms differ and what too much can do. Then check a label against the numbers.
Pick a vitamin
Cobalamin
Vitamin B12
Adult need
2.4 mcg
Upper limit
None set
Label Daily Value
2.4 mcg
Main concern
Running low
2.4 mcg a day for adults, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding.
What it does
B12 is needed to build and protect nerves, make healthy red blood cells and make DNA. The body stores about 1 to 5 mg, which is why a deficiency can take years to show.
Where it comes from in food
B12 is only found naturally in animal foods. Three ounces of clams has 17 mcg, salmon 2.6 mcg, canned light tuna 2.5 mcg and 85% lean ground beef 2.4 mcg. A cup of 2% milk has 1.3 mcg, a 6 ounce yogurt 1.0 mcg and an egg 0.5 mcg. Fortified cereals and fortified nutritional yeast are the plant-based sources.
When to take it
The fact sheet gives no timing rule. B12 in supplements is already free, so people who struggle to release B12 from food still absorb it.
What too much can do
No upper limit is set, because B12 has a low potential for harm even at large doses.
Also worth knowing
Running low is the real risk. Signs include tiredness, a sore tongue, pale skin, a racing heart and numbness or tingling in the hands and feet, and the nerve symptoms can happen without anemia. Metformin and acid reducers such as omeprazole can lower B12, as can older age, stomach or bowel surgery and a vegan diet.
Compare the forms: Vitamin B12
| Cyanocobalamin | Methylcobalamin | Hydroxocobalamin | Sublingual (under the tongue) | |
|---|---|---|---|---|
| What it is | The most common form in supplements, and one of the two prescription injections. | One of the two forms the body actually uses. | Sold in some supplements, and the other prescription injection alongside cyanocobalamin. | Tablets or lozenges that dissolve under the tongue, sold in any of the forms here. |
| In the body | The body converts it to the active forms. | No evidence shows it is absorbed better than other forms. | The body converts it to the active forms. | Studies find no difference in how well it works compared with a tablet you swallow. |
| Worth knowing | In Crohn’s disease, 1,000 mcg a day by mouth may work as well as injections. | In people after gastric bypass, 1,000 mcg a day by mouth worked about as well as injections. | Injections are used when the body cannot absorb B12, as with pernicious anemia. | Useful if swallowing pills is hard. It does not get more B12 in. |
Which absorbs best? The NIH fact sheet finds no evidence that any form absorbs better than another, methylcobalamin included, and under-the-tongue tablets and lozenges work about as well as ones you swallow. What changes absorption is the dose: about half of a 1 to 2 mcg dose is absorbed, about 2% of 500 mcg, and about 1.3% of 1,000 mcg. A prescription nasal gel absorbs about as well as a tablet, around 2% of a 1,000 mcg dose in one small study. Adenosylcobalamin, the body’s other active form, is also sold, with the same caveat.
Check a label: Vitamin B12
Percent Daily Value
41,667%
Adult upper limit
None set
Share of the limit
No limit
1,000 mcg sounds huge at 41,667% of the Daily Value, but only about 1.3% of a dose this size is absorbed, roughly 13 mcg. No upper limit is set.
This is education, not medical advice
B vitamin pills are dietary supplements, and the numbers here are general adult guidance from the NIH. They are not a diagnosis, and only a blood test can show whether you are low. If you take prescription medicines, are pregnant, have kidney disease, or have had weight loss surgery, check with your care team before starting anything new, and tell them about every supplement you take.
All eight at a glance
| Vitamin | Adult need | Upper limit | Daily Value |
|---|---|---|---|
| 1.1 to 1.2 mg | None set | 1.2 mg | |
| 1.1 to 1.3 mg | None set | 1.3 mg | |
| 14 to 16 mg | 35 mg | 16 mg | |
| 5 mg | None set | 5 mg | |
| 1.3 to 1.7 mg | 100 mg | 1.7 mg | |
| 30 mcg | None set | 30 mcg | |
| 400 mcg DFE | 1,000 mcg | 400 mcg DFE | |
| 2.4 mcg | None set | 2.4 mcg |
Only three B vitamins have an upper limit: niacin, B6 and folic acid. The niacin and folic acid limits count supplements and fortified foods only, while the B6 limit counts food too. The other five have no limit because no harm has been shown at high intakes, which is different from saying every dose is useful. Tap a name to open its card above.
B vitamins and a GLP-1
Eating less means less coming in
A 2025 advisory from four lifestyle medicine, nutrition and obesity societies notes that people on GLP-1 medications eat 16% to 39% fewer calories in studies, and that a drop that size can leave vitamins and minerals short, especially below about 1,200 calories a day for women and 1,800 for men. Thiamine (B1) and B12 are on its list of nutrients of concern. It suggests clinicians consider supplements for at-risk nutrients, with B12 or a multivitamin among the examples, at doses fitted to the person.
Vomiting that keeps going, and thiamine
The body holds only a small store of thiamine and uses it quickly. A 2026 systematic review found six published case reports of Wernicke encephalopathy, a brain emergency caused by thiamine deficiency, in people taking semaglutide for obesity. All six had long-running stomach side effects and substantial weight loss first, and three were vomiting. One recovered fully, four were left with lasting memory and thinking problems, and one died. The authors describe it as rare, and they urge clinicians to watch for thiamine deficiency when stomach side effects persist or weight drops fast.
Confusion and abnormal eye movements were common signs in those reports. If you have been vomiting for days or cannot keep food down, call your prescriber that day and ask about thiamine. Confusion, unsteadiness or trouble moving your eyes on top of vomiting needs emergency care, because treatment is thiamine given by injection or IV, and speed matters.
Metformin and B12
Metformin is a first-line medicine for type 2 diabetes and prediabetes, and some people take it alongside a GLP-1. The NIH notes it can reduce B12 absorption and lower B12 in the blood. The American Diabetes Association’s 2026 Standards of Care recommend considering periodic B12 checks for anyone on long-term metformin, especially with anemia or nerve symptoms like numbness and tingling. The same section says people who have taken metformin for more than 4 years, or who have other risks such as a vegan diet or past stomach or small bowel surgery, should be checked for B12 deficiency every year.
Be careful with compounded GLP-1 shots that contain B12
Some compounded semaglutide comes with vitamin B12 mixed into the same vial, and the FDA has said it knows some compounders add it. The FDA has also warned that people have overdosed on compounded semaglutide because of dosing errors, and some ended up in the hospital. In those reports people drew up 5 to 20 times the intended dose, often because the dose was written in units for an insulin syringe, or the math converting milligrams to units was wrong. A poison control center described the same problem: a vial with no concentration on the label and a dose given in units and milliliters instead of milligrams.
The harm in these reports came from the semaglutide dose, and having B12 in the vial does not make it safer. If your GLP-1 comes from a compounding pharmacy, be careful: ask for your dose in milligrams and in units, have the pharmacist show you how to draw it up, and call your prescriber or Poison Control at 1-800-222-1222 if you think you took too much.
If a clinic offers B12 shots
If a clinic or med spa offers you B12 shots, on their own or alongside a GLP-1 or a compounded version of one, here is what is known. A B12 injection is a real prescription treatment, used when the body cannot absorb B12 well, as with pernicious anemia. For someone whose B12 is normal, Cleveland Clinic says the shots themselves are not associated with weight loss, and the NIH says B12 does not appear to improve performance or endurance without a deficiency.
If you are low, a blood test will show it, and you have options. A Cochrane review of three small, low-quality trials found that 1,000 to 2,000 mcg of B12 by mouth brought blood levels back about as well as injections. Your prescriber can help you choose.
Talk to your care team first if this is you
You take metformin, or an acid reducer
Metformin can lower B12 absorption and blood levels, and so can acid reducers such as omeprazole, lansoprazole and cimetidine. Ask whether a B12 test makes sense, and how often.
You take a seizure medicine
Several seizure medicines, including phenytoin, carbamazepine and valproate, can lower B6 and folate, and some also lower biotin. Folate supplements can lower some of these drugs in the blood, and so can 200 mg a day of B6, so changes need a prescriber who knows.
You take methotrexate or sulfasalazine
Methotrexate works against folate, and folate can interfere with it when it is used for cancer. Sulfasalazine blocks folate absorption. Both are reasons to ask before adding folic acid.
You have diabetes and are thinking about niacin
High-dose nicotinic acid can raise blood sugar, most often at 1.5 g a day or more, and diabetes medicines may need adjusting. Doses that high are a prescription-level decision.
You are pregnant or could become pregnant
Folic acid at 400 mcg a day is the standing advice. If you are using B6 for nausea, the doses can come close to the upper limit, so the NIH advises checking with your doctor first.
You have a blood test coming up
Biotin can make thyroid tests, troponin and other results read wrong. Tell whoever orders the test, and ask whether to pause it.
You have had weight loss surgery, drink heavily, or cannot stop vomiting
Each of these raises the risk of low thiamine, and bariatric surgery also raises the risk of low B12. Vomiting that goes on for days on a GLP-1 is a same-day call to your prescriber.
A B-complex or a single vitamin
How the doses differ
A B-complex puts all eight in one product, often at higher doses than a multivitamin. Single-vitamin products tend to carry the most of one. B12 shows the spread: multivitamins typically have 5 to 25 mcg, B-complex products 50 to 500 mcg, and B12-only supplements 500 to 1,000 mcg, against a daily need of 2.4 mcg. Pantothenic acid runs from about 10 mg in a multivitamin to 1,000 mg in a B-complex or single product, against a need of 5 mg, and some niacin-only products carry 500 mg or more.
When a single vitamin fits
A single product makes sense for a specific, known gap: low B12 on a blood test, a medicine that drains one vitamin, or folic acid before pregnancy. It gives a clear dose of the one thing you need without stacking up the others.
When a broader product fits
When the issue is simply eating much less, the GLP-1 nutrition advisory names a multivitamin-mineral as one option to consider, at appropriate doses. Every NIH fact sheet puts food first and treats supplements as the backup when food alone falls short, so a plate with protein foods, beans, greens and whole or enriched grains still does most of the work.
Watch the overlap
If you take more than one product, such as a multivitamin plus a B-complex, the same vitamins can show up in each. Add each line across everything you take before comparing it with the upper limits above.
Reading the label
What percent Daily Value means
The FDA sets one Daily Value per vitamin for adults and children 4 and older, so labels can be compared. 100% means one Daily Value, and a food with 20% or more counts as a high source. B vitamin supplements often list 1,000% or more, and for the five with no upper limit, a big percentage alone is not a danger sign.
A big percentage is not what you absorb
A 1,000 mcg B12 tablet is about 41,700% of the Daily Value, but only about 1.3% of a dose that size is absorbed, roughly 13 mcg. Riboflavin absorption tops out around 27 mg per dose, and thiamine absorption falls off above about 5 mg.
Where the percentage does matter
Check three lines against their limits. B6 at 100 mg is about 5,900% of the Daily Value and already at the US limit. Niacin at 35 mg is about 220%, and nicotinic acid can cause flushing near that point. Folic acid at 1,000 mcg is the limit.
The folate line has two numbers
Folate is listed in mcg DFE, and when folic acid has been added, its plain mcg appear in parentheses, for example 680 mcg DFE (400 mcg folic acid). The upper limit applies to the number in parentheses. 1 mcg of folic acid taken with food counts as 1.7 mcg DFE.
Read the form, too
The ingredient name tells you more than the B number. Niacin as nicotinic acid flushes and niacinamide does not. Folate may be folic acid or L-methylfolate. B12 may be any of four cobalamins, and for B12 the form matters less than the amount.
Sources
14 references
- NIH Office of Dietary Supplements. Thiamin: Fact Sheet for Health Professionals. Updated February 9, 2023. ods.od.nih.gov. Source of the thiamine role, small body store and short half-life, adult RDAs, the 1.2 mg Daily Value, food amounts, supplement forms including benfotiamine and the 1.5 mg multivitamin figure, the absence of an upper limit and the 5 mg absorption point, deficiency signs, Wernicke encephalopathy, the groups at risk including diabetes and bariatric surgery, the benfotiamine neuropathy trials, the UK emergency department guidance on oral thiamine hydrochloride, and furosemide.
- NIH Office of Dietary Supplements. Riboflavin: Fact Sheet for Health Professionals. Updated May 11, 2022. ods.od.nih.gov. Source of the riboflavin role, RDAs, 1.3 mg Daily Value, food amounts and cooking losses, the 27 mg absorption ceiling, the two supplement forms, no upper limit and the 400 mg safety observation, discolored urine, and the migraine findings and guideline ratings.
- NIH Office of Dietary Supplements. Niacin: Fact Sheet for Health Professionals. Updated November 18, 2022. ods.od.nih.gov. Source of NAD and the 400 enzymes, tryptophan conversion, RDAs in niacin equivalents, the 16 mg Daily Value, food amounts, the forms (nicotinic acid, niacinamide, inositol hexanicotinate and extended release) and how they differ, the 500 mg single products, flushing at 30 to 50 mg and how to reduce it, effects at 1,000 to 3,000 mg including liver damage, the 35 mg upper limit and what it covers, the statin trials, and the blood sugar caution for diabetes medicines.
- NIH Office of Dietary Supplements. Pantothenic Acid: Fact Sheet for Health Professionals. Updated May 1, 2026. ods.od.nih.gov. Source of the coenzyme A role, adequate intakes, the 5 mg Daily Value, food amounts and processing losses, calcium pantothenate and pantethine, the 10 mg to 1,000 mg supplement range, the pantethine lipid trials, no upper limit, and diarrhea at about 10 g a day.
- NIH Office of Dietary Supplements. Vitamin B6: Fact Sheet for Health Professionals. Updated June 16, 2023. ods.od.nih.gov. Source of the B6 role, RDAs, the 1.7 mg Daily Value, food amounts, pyridoxine hydrochloride and PLP and their similar absorption, nerve damage at 1 to 6 g a day for 12 to 40 months and reports under 500 mg, the 100 mg US upper limit, the 2023 European 12 mg limit, the ACOG pregnancy nausea guidance, and the seizure medicine interactions.
- NIH Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals. Updated January 10, 2022. ods.od.nih.gov. Source of the biotin role, adequate intakes, the 30 mcg Daily Value, food amounts and raw egg white, full absorption at up to 20 mg, typical intakes of 35 to 70 mcg, the hair and nail evidence including the 2.5 mg nail studies, no upper limit, the lab test interference including the single 10 mg dose, the 10 mg a day study, troponin and the FDA-reported death, and the anticonvulsant interaction.
- NIH Office of Dietary Supplements. Folate: Fact Sheet for Health Professionals. Updated November 30, 2022. ods.od.nih.gov. Source of the folate role, RDAs in DFE and the conversion factors, the 400 mcg DFE Daily Value and how labels list folic acid, food amounts and US fortification since 1998, the absorption of food folate, folic acid and 5-MTHF, the MTHFR and CDC guidance, the 1,000 mcg upper limit and the B12 masking concern, the 400 mcg recommendation for anyone who could become pregnant, the 680 to 1,360 mcg DFE supplement range, and the methotrexate, seizure medicine and sulfasalazine interactions.
- NIH Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025. ods.od.nih.gov. Source of the B12 role and body stores, RDAs, the 2.4 mcg Daily Value, food amounts, the cobalamin forms and the lack of evidence that absorption differs by form, absorption at 1 to 2 mcg, 500 mcg and 1,000 mcg, sublingual and injected forms, supplement dose ranges by product type, deficiency signs, the Cochrane review of oral versus injected B12, the gastric bypass finding, the Crohn’s disease oral dosing note, no upper limit, no performance benefit without deficiency, and the metformin and acid reducer interactions.
- Mozaffarian D, Agarwal M, Aggarwal M, Alexander L, Apovian CM, et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. The American Journal of Clinical Nutrition. 2025;122(1):344–367. doi:10.1016/j.ajcnut.2025.04.023. Source of the 16% to 39% calorie reduction, the 1,200 and 1,800 calorie thresholds, B1 and B12 among the nutrients of concern, and the suggestion to consider supplements for at-risk nutrients including B12 or a multivitamin-mineral.
- Bidesie J, Oudman E. Wernicke’s encephalopathy following semaglutide treatment for obesity: a systematic PRISMA review of case-based evidence. Obesity (Silver Spring). 2026;34(Suppl 2):64–69. doi:10.1002/oby.70256. Source of the six case reports, the prolonged stomach symptoms and weight loss in all six, vomiting in three, the outcomes, the common signs, the authors’ description of it as rare, and their call for vigilance and prompt thiamine by injection or IV.
- American Diabetes Association Professional Practice Committee for Diabetes. 3. Prevention or delay of diabetes and associated comorbidities: Standards of Care in Diabetes–2026. Diabetes Care. 2026;49(Suppl 1):S50–S60. doi:10.2337/dc26-S003. Source of recommendation 3.10, periodic B12 checks with long-term metformin especially with anemia or peripheral neuropathy, and the yearly check after more than 4 years or with other risks such as a vegan diet or past stomach or small bowel surgery.
- Cleveland Clinic. Vitamin B12 Injections: What They Can and Can’t Do. March 5, 2024. health.clevelandclinic.org. Source for B12 shots not being associated with weight loss in people with normal B12, and a Cleveland Clinic family physician’s statement that there is little evidence they work for that purpose.
- US Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 26, 2024. fda.gov. Source for overdoses and hospital visits from compounded semaglutide dosing errors, doses 5 to 20 times the intended amount, unit and syringe confusion, and compounders adding cyanocobalamin (vitamin B12).
- Lambson JE, Flegal SC, Johnson AR. Administration errors of compounded semaglutide reported to a poison control center: case series. Journal of the American Pharmacists Association. 2023;63(5):1643–1645. doi:10.1016/j.japh.2023.06.017. Source for a vial without a labeled concentration and a dose given in units and milliliters instead of milligrams.
Coaching
Want help eating well on less?
Supplements are the backup plan. Coaching helps you build meals that cover the basics with the appetite you have now, and work out which questions to bring to your prescriber.
Work with me →