Thiamine side effects and how to avoid them

In this article:

• Common side effects of thiamine
• Serious side effects of thiamine
• How long do thiamine side effects last?
• What are the long-term side effects of thiamine?
• Interactions
• Contraindications
• How to avoid thiamine side effects
• Treating side effects

In this article

• Common side effects of thiamine
• Serious side effects of thiamine
• How long do thiamine side effects last?
• What are the long-term side effects of thiamine?
• Interactions
• Contraindications
• How to avoid thiamine side effects
• Treating side effects

Thiamine, also known as thiamin or vitamin B1, is a water-soluble vitamin found naturally in some foods, added to foods, and sold as a supplement. Brand names may include Benerva, Tyvera, Betalin S, ThiaDose, and ThiamEss. “Thiamine is essential for energy metabolism because it supports neuronal activity and transforms carbs into useful energy,” says Iven Gonzalez, DDS, owner of Sun City Dental in El Paso, Texas. Synthetic or manmade thiamine in the form of oral tablets or injections is used to treat or prevent vitamin B1 deficiency. 

Common side effects of thiamine

Dr. Gonzalez lists the following as the most common side effects of thiamine supplementation:

• Flushing (sudden reddening of skin)
• Nausea 
• Sweating
• Itching

“In most cases, side effects appear shortly after treatment and go away quickly,” he says.

Serious side effects of thiamine

The only known severe side effect of thiamine is anaphylaxis. Anaphylaxis is a life-threatening allergic reaction that can occur within minutes of exposure to an allergen, such as food, insect stings, or medicines. A case study describes a 44-year-old patient admitted to the hospital with alcohol intoxication who developed cardiac arrest after experiencing anaphylaxis. The reaction occurred shortly after he received an intravenous (IV) thiamine infusion. 

The following symptoms of anaphylaxis may occur within 30 minutes of administering IV thiamine:

• Shortness of breath
• Hives
• Difficulty swallowing
• Red rash
• Chest tightness
• Abdominal pain
• Cramps
• Vomiting
• Wheezing
• Feeling of doom or dread
• Diarrhea

Severe signs of anaphylaxis that require immediate medical attention are:

• Increased heart rate
• Lightheadedness or confusion due to a sudden drop in blood pressure
• Sudden muscle weakness
• Swelling of the lips, tongue, or throat
• Unconsciousness
• Cardiac arrest

If you’re experiencing symptoms of anaphylaxis, please visit your nearest emergency room or dial 911 for immediate care. The medical condition is treatable with an epinephrine autoinjector (such as an EpiPen), along with oxygen therapy and fluids, if you’re having difficulty breathing.

It’s important to emphasize that this side effect of thiamine is extremely rare. In a study involving 8,606 intravenous thiamine administration across 2,595 patients, researchers observed only 18 possible and eight probable injection site reactions. However, there were no instances of anaphylaxis among these patients.

High-dose thiamine side effects

According to Dr. Gonzalez, excessive consumption of thiamine may cause gastrointestinal distress, such as nausea, vomiting, and diarrhea. “However, high dosages are usually eliminated in the urine, reducing the danger of poisoning,” he explains. 

How soon do thiamine side effects start?

Side effects of thiamine can occur shortly after taking the dietary supplement. “If someone does experience side effects, they typically show up shortly after taking the supplement, within minutes to an hour, particularly if it is given by injection,” Gail Clifford, MD, MMM, CPE, FACP, FHM, a hospitalist and medical consultant at ARRC LED. “For oral thiamine, any side effects would likely appear the same day.”

How long do thiamine side effects last?

Dr. Clifford says that most side effects of thiamine are temporary. “Mild symptoms like nausea or flushing generally fade within a few hours,” she explains. “Severe reactions, if they occur, are medical emergencies and would be treated right away.”

What are the long-term side effects of thiamine?

According to Dr. Clifford, there are no known long-term side effects of thiamine at standard or high dosages. “In fact, the bigger issue we tend to see in healthcare is not too much thiamine, but too little, especially in people with chronic alcohol use or malabsorption problems,” she says.

Dr. Clifford adds that because long-term side effects are very rare, healthcare providers tend to focus more on the risk factors of thiamine deficiency, such as alcoholism, chronic illnesses, certain gastrointestinal disorders, and long-term use of diuretics. “These conditions make it harder for the body to absorb or retain enough B1,” she explains.

Thiamine interactions

Below are some potential interactions that could occur while taking this dietary supplement:

• Diuretics: Diuretics, also known as water pills, help reduce fluid buildup in your body. “Diuretics like furosemide can cause your body to lose more thiamine through urine,” Dr. Clifford says.
• Betel nut: Betel nut is a seed of the fruit of the areca palm tree. The stimulant drug can cause chemical changes to thiamine, making it less effective in the body.
• Alcohol: Alcohol is one of the main drug interactions of thiamine, Dr. Clifford says. “It can reduce both absorption and storage [of thiamine].”
• Raw freshwater fish and shellfish: These seafoods contain certain chemicals that can destroy thiamine. However, if the food is cooked, they won’t interact with the dietary supplement.
• Caffeine-containing foods and drinks: Tannins are chemical compounds within caffeine products. When tannins react with thiamine, they can change it into a form that’s difficult for the body to absorb. Ultimately, this could lead to thiamine deficiency over time.

“These interactions are more likely to lead to deficiency over time rather than cause immediate harm,” Dr. Clifford says.

Thiamine contraindications

The main contraindication for thiamine is an allergy to thiamine itself. “Those who are allergic to thiamine, however, should not take supplements,” Dr. Gonzalez says. 

Aside from potential allergies to thiamine, you should consult with your healthcare provider before taking the supplement if:

• You drink alcohol
• You have difficulty absorbing food
• You’re receiving dialysis treatments
• You’re pregnant or plan to become pregnant
• You’re breastfeeding

You should also notify your healthcare provider of any over-the-counter and prescription medications and other supplements you’re currently taking, as they may need to adjust the doses or closely monitor you for potential side effects.

Some people worry that vitamin B1 supplements may cause weight gain. However, this is a misconception as weight loss is actually a symptom of thiamine deficiency. “Thiamine helps your body use food for energy more efficiently,” Dr. Clifford explains. “In deficient people, supplementing with thiamine may lead to a better appetite or improved energy, which could result in healthy weight gain—but that is not a side effect. It is a sign that the body is working better.”

It’s also important to note that while you cannot overdose on thiamine, high doses of the supplement may cause stomach upset. A high amount of thiamine could be anything beyond the recommended 50 to 100 milligrams (mg) daily for oral supplements. However, higher doses have been used in people with severe deficiencies. 

How to avoid thiamine side effects

You can lower your risk of experiencing unwanted side effects from thiamine supplements by taking the following steps:

1. Follow the dose prescribed by your healthcare provider. For children prescribed thiamine, the dosage will be based on their weight.
2. Limit alcohol consumption while taking thiamine, as it can interfere with the absorption and effectiveness of the supplement.
3. Take oral tablets by mouth, either with or without food, at your preferred time of the day. Swallow tablets with a glass of water. If you have difficulty swallowing it, you can break it in half (if the tablet is scored) and take both halves separately.
4. Continue taking the supplement until your vitamin B1 levels have returned to normal, which may require follow-up lab tests to monitor your progress.
5. If you miss a dose, take it as soon as you remember—unless it’s nearly time for your next dose. In that instance, just skip the missed dose and take the next one at your regular time.

If you have questions or concerns about taking thiamine, please consult your provider or pharmacist for medical advice.

How to treat side effects of thiamine

If you experience side effects like nausea or skin itching after taking thiamine, Dr. Clifford says these symptoms will typically go away on their own. “If someone feels flushed or gets a headache, resting and drinking water can help,” she says. “But if someone shows signs of an allergic reaction, like swelling, trouble breathing, or hives, that’s a medical emergency that needs immediate attention.”

Bottom line: Is thiamine safe?

Thiamine is generally a safe supplement for treating vitamin B1 deficiency. While the supplement can cause side effects such as flushing and sweating, these typically subside within a short period. In rare cases, thiamine injections may cause severe allergic reactions, which require immediate medical attention. Be sure to follow your healthcare provider’s instructions while taking thiamine to lower your risk of adverse reactions.