Stack essential · Deficiency fix

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.

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Dose

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When to take

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Signs you actually need this

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Signs it's working

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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)

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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.