From the Guidelines
Oral thiamine is not equivalent to intravenous (IV) thiamine for treating acute deficiency states, and IV thiamine is the preferred route for patients with suspected or confirmed thiamine deficiency, particularly in severe cases. According to the most recent guideline from 2022, thiamine can be administered orally, enterally, or IV, but the IV route is the most efficient for providing high doses, such as 3 x 100-300 mg per day 1. This is because oral thiamine has limited absorption and may not be sufficient for treating acute deficiency states, especially in patients with malabsorption issues.
For patients admitted to emergency or intensive care, IV thiamine (100-300 mg/day) should be prescribed without hesitation from admission for 3-4 days 1. In patients with suspected or confirmed thiamine deficiency, IV thiamine is strongly preferred due to its higher bioavailability and absorption rate compared to oral thiamine. However, oral thiamine at doses of 100mg daily can be appropriate for maintenance therapy after IV treatment or for prevention in at-risk patients without acute deficiency.
The key differences in efficacy between oral and IV thiamine stem from thiamine's absorption mechanism in the gastrointestinal tract, which becomes saturated at low doses, limiting the amount that enters circulation. Additionally, many patients with thiamine deficiency have malabsorption issues that further reduce oral bioavailability. For this reason, clinicians should not substitute oral for IV thiamine in acute settings, as inadequate treatment of thiamine deficiency can lead to permanent neurological damage. The consensus on this recommendation is strong, with a grade of recommendation of 0 and a consensus of 88% 1, and a grade of recommendation B with a strong consensus of 92% for patients admitted on the ward with any suspicion of reduced food intake or high alcohol consumption 1.
From the FDA Drug Label
Dosage and Administration “Wet” beriberi with myocardial failure must be treated as an emergency cardiac condition, and thiamine must be administered slowly by the IV route in this situation (see WARNINGS). In the treatment of beriberi, 10 to 20 mg of thiamine hydrochloride are given IM three times daily for as long as two weeks. (See WARNINGS regarding repeated injection of thiamine.) An oral therapeutic multivitamin preparation containing 5 to 10 mg thiamine, administered daily for one month, is recommended to achieve body tissue saturation.
The FDA drug label does not directly compare the equivalence of oral and IV thiamine, but it provides different dosing recommendations for each route of administration.
- For IV administration, the label recommends doses ranging from 25 mg to 100 mg.
- For oral administration, the label recommends a daily dose of 5 to 10 mg as part of a multivitamin preparation. Since the label does not provide a direct comparison of the two routes, no conclusion can be drawn about their equivalence 2.
From the Research
Equivalence of Oral and IV Thiamine
- The equivalence of oral and IV thiamine is not directly addressed in the provided studies, but they do discuss the importance of thiamine supplementation in various clinical settings 3, 4, 5, 6, 7.
- A study from 1996 found that 3 mg of thiamine hydrochloride added to total parenteral nutrition (TPN) solutions was adequate to maintain normal thiamine status in patients with compromised intestinal thiamine absorption 3.
- Another study from 2018 highlighted the importance of considering thiamine deficiency in critically ill patients and noted that intravenous thiamine can correct lactic acidosis, improve cardiac function, and treat delirium 4.
- A 2015 study discussed the role of thiamin in clinical practice, including its absorption, digestion, and metabolism, and provided evidence-based practice suggestions for the prevention and treatment of thiamine deficiency 5.
- A 2022 study described the clinical presentation, diagnosis, and treatment of thiamine deficiency in a tertiary hospital, and found that prompt parenteral supplementation was required to improve symptoms and avoid chronic complications 6.
- A 2015 study examined thiamine prescribing practices in university-affiliated hospitals and found that parenteral thiamine accounted for a majority of thiamine prescriptions, but oral thiamine was still commonly prescribed 7.