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
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
When to take
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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