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Thiamine (Vitamin B1)

GLP-1 users frequently run B1-deficient because the appetite drop kills food intake before nutrient targets are met.

What it is

Vitamin B1. Cofactor in glucose metabolism and nerve function. Deficiency shows up as fatigue, brain fog, peripheral neuropathy. Body stores are small, clinical depletion happens in weeks, not months.

Why peptide users take it

Tirzepatide, retatrutide, and semaglutide users routinely cut food intake by 30–50%. Thiamine is the first water-soluble vitamin to drop because it's not stored long. Several published case reports of Wernicke's encephalopathy in GLP-1 users with extreme appetite loss. Most users won't hit clinical deficiency, but subclinical (fatigue, brain fog) is common.

Pairs with these peptides

Retatrutide
Triple-agonist appetite suppression is the strongest in class, depletion risk highest
Tirzepatide
Dual-agonist appetite suppression; subclinical fatigue common in month 2+
Semaglutide
Long-term users at risk; case reports exist

How it actually works

Thiamine is converted to thiamine pyrophosphate (TPP) in the liver. TPP is the cofactor for pyruvate dehydrogenase, the enzyme that lets your cells turn glucose into ATP (energy). Without enough TPP, glucose breakdown stalls, lactate builds up, and the brain (which runs almost entirely on glucose) feels it first. That's why deficiency shows up as fatigue and brain fog before anything more dramatic.

Dose

Members only

When to take

Members only

Signs you actually need this

  • Persistent low-grade fatigue that worsens through the day
  • Brain fog or trouble concentrating, especially mid-afternoon
  • Tingling or numbness in feet/hands (early peripheral neuropathy)
  • Unexplained anxiety or irritability
  • Loss of appetite even on lower GLP-1 doses

Signs it's working

  • Mid-afternoon energy improves within 2–3 weeks
  • Brain fog lifts (subtle but real)
  • Tingling/numbness, if present, slowly resolves over 4–8 weeks on benfotiamine
  • Cognitive endurance through long workdays improves

Common mistakes

  • Megadosing without ruling out other deficiencies (B12, iron) first, symptoms overlap heavily
  • Stopping the moment fatigue lifts, you'll relapse if GLP-1 dose stays high. Stay on it through the cycle.
  • Buying a generic 'B-complex', most have only 10–25 mg B1, not the 100 mg+ relevant for GLP-1 users
  • Confusing thiamine HCl (cheap, water-soluble) with benfotiamine (fat-soluble, far better for neuropathy)

Where to buy

Editorial picks. Brand selection follows the quality criteria (USP/NSF or third-party COA where possible). Affiliate links , see the disclosure at the bottom of /supplements.

Caveats

  • Excess B1 is excreted in urine, overdose is rare but not zero.
  • If you have actual Wernicke symptoms (confusion, ataxia, vision changes), this is an ER visit, not an Amazon order.

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Educational reference only. Not medical advice. Talk to a doctor before starting any supplement, especially if you take prescription medications or have an underlying condition.
Weeks
How long body thiamine stores last — far shorter than most vitamins

What it is

Vitamin B1 — the nutrient behind beriberi and, more urgently, Wernicke's encephalopathy, a medical emergency where delay increases the risk of irreversible neurological injury, Korsakoff syndrome or death. That emergency use is the reason this entry exists; the wellness marketing is a distant second.

How it works

Thiamine is the cofactor that lets cells convert carbohydrate into energy. Nervous tissue and heart muscle are the most metabolically demanding and fail first — which is why deficiency presents as neurological and cardiac emergencies rather than as vague tiredness. Body stores last only weeks, so deficiency develops faster than for most vitamins.

Which form to buy

Thiamine HCl / mononitrateStandard use

The common cheap forms in supplements and fortified foods.

Adequate for ordinary needs.

BenfotiamineDiabetic neuropathy research

An S-acyl thiamine derivative with higher oral bioavailability. The 2026 BOND trial (600 mg/day for 12 months) found NO significant benefit across nerve, clinical or quality-of-life outcomes.

Better absorbed; the outcome evidence is not there.

IV/IM thiamineMedical emergencies

The route used for suspected Wernicke's — oral absorption is too limited and too slow for that situation.

Hospital treatment, not a supplement decision.

Food (pork, legumes, fortified grains)Everyone

Fortification made deficiency uncommon in many countries.

Sufficient for most people.

What it actually does, graded

Wernicke's encephalopathy and beriberi are its recognised medical usesStrong

A genuine medical emergency use. Prompt treatment substantially reduces the risk of lasting injury, though it does not guarantee full recovery.

Corrects deficiency in alcohol dependenceStrong

Alcohol impairs absorption and storage; supplementation is standard care in that setting.

Deficiency after bariatric surgery or prolonged vomitingStrong

A recognized and clinically important risk group — including severe pregnancy sickness.

Diabetic neuropathy (benfotiamine)Limited

Mixed earlier results, and the 12-month BOND randomized trial found no significant benefit on any measured outcome.

Energy in replete peopleNone shown

The "B1 for energy" pitch has no support without deficiency.

Pros and cons

Pros
  • Very cheap and extremely safe
  • The emergency use genuinely reduces the risk of lasting brain injury
  • Important for identifiable risk groups people rarely think about
Cons
  • The consumer energy claim is unsupported
  • Oral absorption is limited, which is why emergencies need injection
  • Deficiency is easy to miss because symptoms look like many other things

What to expect

  • In genuine deficiency, response can be dramatic — though rapid improvement is most characteristic of the eye signs; confusion and gait problems may recover slowly or incompletely.
  • If replete: nothing.

Interactions

CautionAlcoholThe single most important context — alcohol impairs thiamine absorption, storage and activation, which is why deficiency clusters here.
MonitorLoop diuretics (furosemide)Long-term use can increase thiamine loss.
CautionGlucose-containing IV fluids in deficiencyIn at-risk patients, parenteral thiamine should be given before or alongside carbohydrate where feasible, and prolonged glucose administration without thiamine avoided. ⚠️ NEVER delay urgent dextrose for hypoglycaemia in order to give thiamine first — hospital clinicians manage this sequencing.

Is it for you?

Probably worth it if
  • Heavy alcohol use, post-bariatric surgery, or prolonged vomiting — the genuine risk groups
  • Persistent vomiting in pregnancy, as a clinician conversation
Probably skip it if
  • You eat a normal fortified diet and want energy — deficiency is uncommon and repletion does nothing extra

Who should avoid it

  • Nobody in particular — thiamine is remarkably safe. The real error is treating suspected deficiency casually rather than urgently.

Research on this supplement

Papers about this supplement, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

  • What is the optimum thiamine dose to treat or prevent Wernicke's encephalopathy or Wernicke-Korsakoff syndrome? Results of a randomized controlled trial
    Alcohol Clin Exp Res, 2022
  • Effect of Vitamin C, Thiamine, and Hydrocortisone on Ventilator- and Vasopressor-Free Days in Patients With Sepsis: The VICTAS Randomized Clinical Trial
    JAMA, 2021
  • Effect of Vitamin C, Hydrocortisone, and Thiamine vs Hydrocortisone Alone on Time Alive and Free of Vasopressor Support Among Patients With Septic Shock: The VITAMINS Randomized Clinical Trial
    JAMA, 2020

Evidence base

A
3,906 research papers
Strong human evidence: either a phase 3 or 4 trial report alongside review-level evidence, or ten or more randomised controlled trial reports.

Where next

The full supplement shelfEvery staple, tiered by evidence — not by margin.→What people stack these aroundThe compound index, evidence-graded.→Ask Coach about Thiamine (Vitamin B1)Answers with the research it stands on.→
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