Thiamine (Vitamin B1): Benefits, Dosage, Benfotiamine, and What the Trials Really Found

Thiamine is vitamin B1. Your cells need it to turn carbohydrate into energy.

In human trials the best evidence is for mood, reaction time and fatigue. For blood sugar the trials came back negative, and I have to correct some of my own posts on that.

Thiamine benefits: what the studies show

• Mood and reaction time. 120 young women took 50 mg of thiamine or a placebo every day for 2 months. A better thiamine status went with feeling more clearheaded, composed and energetic. Reaction times got faster. Memory did not change. These women had adequate thiamine by the usual standard before they started. (Benton 1997)
• Depression, as an add-on. 51 inpatients with major depression all received an SSRI. Half also got thiamine and half a placebo. The thiamine group improved faster and was ahead at 6 weeks. At 12 weeks the difference was no longer statistically significant. No side effects were reported. (Ghaleiha 2016)
• Fatigue. 40 patients with quiet inflammatory bowel disease and severe chronic fatigue took 600 to 1,800 mg a day for 4 weeks in a crossover trial. Fatigue fell by 4.5 points on thiamine and rose by 0.75 on placebo. (Bager 2021)
• Gut barrier, in mice. Mice on a high-fat, high-fructose diet got high-dose thiamine for 8 weeks. Their obesity symptoms eased, tight junction function improved, Proteobacteria went down and gut-derived endotoxin fell. (Xia 2025)

In the depression trial thiamine was added to the drug. It did not replace it. Nothing here is a reason to stop a prescription.

Thiamine deficiency symptoms

Deficiency does not look like one thing. It can affect the metabolic, nervous, cardiovascular, respiratory, digestive and muscular systems, which is why it is often misdiagnosed. (Smith 2021)

• Early and vague: irritability and symptoms of depression.
• Severe: Wernicke encephalopathy and Korsakoff psychosis, the brain disorders best known from alcoholism. (Chandrakumar 2018)
• A clue for doctors: unexplained neurological or heart disease, especially with lactic acidosis. (Schostak 2023)

It is not only an alcohol problem.

• Among 400 patients with a BMI of 35 or more, 16.5% met the criteria for clinical thiamine deficiency. Of those, 70% had heart symptoms, 59% had nerve symptoms in the limbs, 14% had gut symptoms and 5% had neuropsychiatric symptoms. (Nath 2017)
• Among 146 US veterans with obesity, 32.2% had a thiamine level below the reference range. (Costello 2025)

If you suspect a real deficiency, that is a medical problem. See a doctor. Do not treat it from a blog.

Thiamine dosage

The official daily requirement for adults is small, about 1.1 to 1.2 mg. The trials used far more:

• 50 mg a day in the mood study.
• 600 to 1,800 mg a day, set by sex and body weight, in the fatigue trial.
• 100 to 900 mg a day in the diabetes trials below.

Plain thiamine is absorbed better than people assume. Healthy subjects took single oral doses of 100, 500 and 1,500 mg of thiamine hydrochloride. Blood levels kept rising with the dose, though less steeply at the top. (Smithline 2012)

More is not always needed. After the fatigue trial, 300 mg a day for 12 weeks did not hold the improvement. (Bager 2022)

What I do: a B complex that contains 20 to 25 mg of prosultiamine, a fat-soluble form, every day. In my view 50 to 300 mg a day of plain thiamine is enough for most people who want to try it.

Thiamine vs benfotiamine

Benfotiamine is a fat-soluble precursor that the body turns into thiamine.

• Absorption. In healthy volunteers, benfotiamine gave about 11 times the plasma thiamine of thiamine hydrochloride, and about twice the active form inside red blood cells. (Xie 2014)
• Nerve pain in diabetes. 165 patients with diabetic polyneuropathy were randomized to 300 or 600 mg of benfotiamine a day or a placebo for 6 weeks. The symptom score improved in the patients who completed the protocol. In the full analysis it just missed significance. (Stracke 2008)
• Early Alzheimer's. 70 people with mild cognitive impairment or mild dementia took benfotiamine or a placebo for 12 months. The main cognitive test was not significantly different. A secondary dementia rating worsened 77% less on benfotiamine. The authors call it safe and potentially useful. (Gibson 2020)

I prefer other fat-soluble forms, such as prosultiamine, allithiamine and TTFD, and I think benfotiamine is fine. That preference is my opinion. One pharmacology review ranks benfotiamine above the other fat-soluble forms for bioavailability. (Loew 1996)

Thiamine side effects

• By mouth: the depression trial reported none. The fatigue trial reported only mild side effects at up to 1,800 mg a day.
• In hospital, into a vein: 989 patients received a 100 mg dose. There were 12 reactions, 1.1%. Eleven were brief local irritation and one was generalized itching. (Wrenn 1989)

Serious allergic reactions to injected thiamine have been reported, which is why that study was done. That is a hospital matter.

The honest part

• Blood sugar. A meta-analysis of 6 trials with 364 adults with type 2 diabetes found that 100 to 900 mg a day of thiamine or benfotiamine, for up to 3 months, did not change HbA1c, fasting glucose or glucose after meals. HDL rose slightly. (Muley 2022)
• The mood study was in women only, and the benefit tracked thiamine status, which is not the same as proof that the pill caused it.
• The depression advantage was gone by week 12.
• The gut study was in mice.

I have posted that B1 should be the standard treatment for diabetes. The pooled human trials do not support that. If you have diabetes, do not change your treatment because of a vitamin.

What I think

My view is that the more carbohydrate you burn, the more thiamine you use. I eat a lot of carbohydrate, so I make sure B1 is covered. I think it works together with B2 and B3, which is why I take a complex and not B1 alone.

I once gave a tired friend 100 mg twice a day. He also started drinking orange juice and eating more carbs, and he felt like a different person within days. I cannot tell you which change did it.

More B vitamin research is in the B vitamin studies. Why I care about burning glucose at all is in Glucose vs fat.

Quick answers

What are the benefits of thiamine?

In 120 young women, 50 mg a day for 2 months was linked to feeling more clearheaded and energetic and to faster reaction times. In 40 patients with inflammatory bowel disease, 600 to 1,800 mg a day reduced chronic fatigue. As an add-on to an SSRI it sped up improvement in depression at 6 weeks.

What is the right thiamine dosage?

The daily requirement is about 1.1 to 1.2 mg. Trials used 50 mg a day for mood and 600 to 1,800 mg a day for fatigue. In a follow-up, 300 mg a day did not maintain the fatigue benefit.

Is benfotiamine better than thiamine?

It is absorbed better. In healthy volunteers benfotiamine gave about 11 times the plasma thiamine of thiamine hydrochloride. Whether that means better results is less clear. Its trials in diabetic nerve pain and early Alzheimer's were mixed.

What are the symptoms of thiamine deficiency?

They vary widely: irritability and low mood, nerve symptoms in the limbs, heart symptoms, gut symptoms and, in severe cases, Wernicke encephalopathy. In one clinic, 16.5% of patients with a BMI of 35 or more had clinical thiamine deficiency.

What are the side effects of thiamine?

Oral trials reported none or only mild side effects at up to 1,800 mg a day. With intravenous thiamine in hospital, 1.1% of 989 patients had a reaction, almost all of them brief local irritation.

Does thiamine lower blood sugar?

Not in the pooled trials. A meta-analysis of 6 trials with 364 adults with type 2 diabetes found no effect of thiamine or benfotiamine on HbA1c or fasting glucose.