Are there any contraindications to supplementing with thiamin 200mg in a patient with a history of Small Intestine Bacterial Overgrowth (SIBO) and malabsorption, who has recently received 1000mg of vitamin B12 (Cobalamin) intramuscularly and is experiencing severe fatigue?

Thiamine 200mg Supplementation is Safe and Likely Beneficial in Your Situation

There are no contraindications to taking thiamine 200mg daily in your context of SIBO, malabsorption, and severe fatigue following B12 injection—in fact, this dose is specifically recommended for patients with malabsorption conditions. 1

Why Thiamine Supplementation Makes Sense for You

Your clinical picture strongly suggests thiamine deficiency as a contributing factor to your fatigue:

• SIBO selectively depletes thiamine because bacteria in the small intestine preferentially consume thiamine for their own metabolism, creating isolated B1 deficiency even when other nutrients remain adequate 1
• Thiamine stores deplete within just 20 days of inadequate intake or malabsorption—far faster than any other B vitamin—making it the first deficiency to manifest clinically in malabsorption conditions 1
• Your B12 was just repleted, but thiamine likely wasn't addressed, creating a mismatch where one deficiency is corrected while another persists and may be causing your ongoing fatigue 1

Safety Profile of 200mg Thiamine

Thiamine has an exceptionally favorable safety profile with no established upper toxicity limit 1:

• Excess thiamine is simply excreted in urine with no accumulation 1
• The only contraindication is a history of allergic reaction to thiamine itself 2
• Doses up to 400mg are well-tolerated; only doses exceeding 400mg may occasionally cause mild nausea, anorexia, or mild ataxia 1
• Your proposed 200mg dose falls well within the safe therapeutic range 1

Evidence Supporting 200mg Dosing

Clinical guidelines specifically recommend 200-300mg daily thiamine for patients with malabsorption conditions 1:

• Post-bariatric surgery patients with prolonged vomiting or poor intake receive 200-300mg daily 1
• Patients with SIBO and malabsorption benefit from higher oral doses (200-300mg) because absorption is compromised 1
• A high-quality randomized controlled trial in IBD patients with malabsorption showed that high-dose oral thiamine (600-1800mg based on weight) significantly reduced chronic fatigue, with 55-75% of patients experiencing meaningful improvement compared to 25-35% on placebo 3

Addressing Your Specific Situation

Your severe fatigue after B12 injection could represent:

1. Unmasked thiamine deficiency: Correcting B12 alone doesn't address concurrent thiamine depletion from SIBO-related malabsorption 1
2. Refeeding-type phenomenon: When one deficiency is corrected, metabolic demands may increase, unmasking other deficiencies 1
3. Ongoing SIBO effects: Bacterial overgrowth continues to consume thiamine even after B12 repletion 1, 4

Practical Recommendations

Start thiamine 200mg orally once daily immediately 1:

• Take it consistently for at least 4 weeks to assess response 3
• Monitor for improvement in fatigue, which typically occurs within 2-4 weeks if thiamine deficiency is contributing 3
• If absorption is severely compromised and oral supplementation fails after 4 weeks, consider discussing IV thiamine (100-300mg) with your physician 1

Address the underlying SIBO 4:

• Treating SIBO is essential before supplementation can be fully effective, as ongoing bacterial overgrowth will continue depleting thiamine 4
• Consider hydrogen/methane breath testing if SIBO diagnosis hasn't been confirmed 4

Monitor other fat-soluble vitamins 4:

• SIBO causes bile salt deconjugation, leading to malabsorption of vitamins A, D, E, and K 4
• Check serum 25-hydroxyvitamin D, retinol, alpha-tocopherol, and vitamin K1 levels every 6 months 4

Critical Pitfall to Avoid

Do not take bile acid sequestrants (cholestyramine, colesevelam) if prescribed for diarrhea, as these will worsen both thiamine and fat-soluble vitamin deficiencies despite controlling symptoms 4

Bottom Line

Thiamine 200mg daily is not only safe but specifically indicated for your situation. The combination of SIBO, malabsorption, and severe fatigue makes thiamine deficiency highly likely, and supplementation at this dose carries essentially no risk while offering substantial potential benefit. 1, 3