I am seeking help or ideas on a problem I seem to have regarding
thiamine (B1).

I have seen countless doctors for issues with a serious fatigue
problem over the past 20 years.

Muscle fatigue / weakness and mental fog are the main complaints.
About 6 years ago one of the doctors prescribed a few vitamins,
including a "standard" multivitamin. A day or so after starting the
regimen I began to feel even worse. After a process of elimination it
was revealed that Vitamin B1, thiamine, was causing me to feel utterly
lousy. The doctor felt that it must simply be the preparation of the
vitamin that I am sensitive to. I disagree, I have tried, sometimes
accidentally, various other preparations and all ultimately have the
same poor results.

It is truly awful how bad I feel after using B1.  I experience
profound muscle aches/pain, extreme weakness, severe mental fog. It
seems to amplify my underlying main complaint!

Some things I have noticed. If I abstain from taking B1 for some time,
and then allow it to be introduced, I can tolerate it for about a day.
It seems to me that would be indicative of possible overload of some
enzyme(s)? Trouble excreting B1, which is possible with kidney issues,
could also be a cause?

I should note that I have taken Benfotiamine and it brought on the
symptoms even faster! It was awful.

It is rather discouraging since it can be helpful in treating fatigue
related problems for so many people and is almost universally regarded
as having no side effects. I could find very few cases where B1
intolerance was even observed.

What in the world could be causing this?

Thanks for taking the time to read this post.

JB