What are the potential side effects of administering 100 milligrams of thiamine (Vitamin B1) intramuscularly (IM) to an adult for supplementation, regardless of deficiency status?

From the Guidelines

Administering 100 mg of thiamine intramuscularly to an adult for supplementation without confirmed deficiency is generally safe with minimal risk of side effects, but oral supplementation is preferred unless there are concerns about absorption issues or severe deficiency requiring rapid correction. The most common side effects of intramuscular thiamine might include mild pain or irritation at the injection site. Rarely, some individuals may experience allergic reactions such as rash, itching, or in very rare cases, anaphylaxis 1. Thiamine is water-soluble, so excess amounts are typically excreted in urine rather than stored in the body, making toxicity uncommon. According to the espen micronutrient guideline, the oral route is adequate in cases of suspicion of chronic deficiency without any acute disease, and the IV route is recommended in cases of acute disease or suspicion of inadequate intake 1. The guideline also suggests that thiamine can be administered orally, enterally, or IV, with the IV route being the most efficient in severe cases, providing 3 x 100-300 mg per day 1.

Some key points to consider when deciding on thiamine supplementation include:

• The severity of potential deficiency symptoms, which can range from mild to severe and involve various systems, including neurological, psychiatric, and cardiovascular systems 1
• The risk of refeeding syndrome, which is associated with increased mortality, particularly in inpatients 1
• The depletion of thiamine reserves as early as 20 days of inadequate oral intake, highlighting the need for early suspicion and recognition of thiamine deficiency 1
• The preference for oral supplementation unless there are concerns about absorption issues or severe deficiency requiring rapid correction, as oral thiamine is generally well absorbed, except in cases of alcohol-related gastritis 1

Overall, while intramuscular thiamine is relatively safe, it is essential to establish a clinical indication before administering any parenteral medication, and oral supplementation is typically adequate and less invasive for most patients without confirmed deficiency.

From the FDA Drug Label

Adverse Reactions An occasional individual may develop a hypersensitivity or life-threatening anaphylactic reaction to thiamine, especially after repeated injection. collapse and death have been reported. A feeling of warmth, pruritus, urticaria, weakness, sweating, nausea, restlessness, tightness of the throat, angioneurotic edema, cyanosis, pulmonary edema, and hemorrhage into the gastrointestinal tract have also been reported. Some tenderness and induration may follow IM use (see WARNINGS). Overdosage Parenteral doses of 100 to 500 mg singly have been administered without toxic effects.

Potential Side Effects:

• Hypersensitivity reactions
• Anaphylactic reactions
• Local reactions at the injection site, such as tenderness and induration Giving 100 mg of thiamine IM to an adult for supplementation may cause side effects, including hypersensitivity or anaphylactic reactions, even if there is no known deficiency 2. However, parenteral doses of 100 to 500 mg singly have been administered without toxic effects 2.

From the Research

Potential Side Effects of Thiamine Supplementation

• Anaphylaxis: A case study reported an instance of anaphylaxis in a 78-year-old female after receiving intramuscular injections of thiamine hydrochloride 3.
• Compartment allergy: The same case study suggested that the patient had a compartment allergy with hypersensitivity to parenteral but not to oral thiamine 3.
• Increased tissue uptake: A study on the effect of intravenous infusions of thiamine found that tissue uptake and elimination of thiamine are dose-dependent, and that slow infusion rates may be superior to rapid infusion rates or bolus injections 4.

Considerations for Thiamine Supplementation

• Thiamine deficiency: Thiamine deficiency can develop in patients secondary to inadequate nutrition, alcohol use disorders, increased urinary excretion, and acute metabolic stress 5.
• Modern thiamine deficiency: A study suggested that the recommended daily allowance (RDA) for thiamine may be insufficient to meet the demands of modern living, and that measurable thiamine deficiency has been observed across multiple patient populations 6.
• High-dose thiamine: A case report found that high-dose thiamine administered intramuscularly led to a rapid and persistent improvement of symptoms in two patients with essential tremor, with no relevant side effects reported 7.

Administration Route Considerations

• Intramuscular injections: The case study on anaphylaxis highlighted the potential risk of allergic reactions with intramuscular injections of thiamine 3.
• Oral intake: The same case study suggested that oral intake of thiamine may be better tolerated due to the uptake mechanism of thiamine in the gastrointestinal system 3.