Where is the data supporting high doses of vitamins for neuropathy?

Probably from drug companies, clinical trials, research papers, etc., but you would need to ask the doctors. You can probably find most of the information using PubMed, Educational Research organizations and even Google Scholar (https://scholar.google.com/)

My non medical opinion/guess is there probably is not a consensus.

@rollo8
When you get bloodwork for deficiencies and toxicities, there are reference ranges that are considered “normal.” If you are found to be deficient or at toxic levels, you would be recommended to supplement at a certain level for a period of time and then retested/adjusted. If toxic levels, you would be recommended to stop any supplements and reduce diet to lower foods causing toxicities. Working with a nutritionist would be able to help guide on diet/supplementation.

I guess what I am really asking is why don't our doctors share this source of information? Widely varying dosages given for supplements by their doctors for members on this page. If there is a consensus on what truly works, be it Rx meds, supplements, or treatments, THEN GET IT TOGETHER and share as a collective of neurologists.
And, while I am on this bandwagon; there should be a dedicated list of common symptoms for early signs of neuropathy, for All primary care providers.

My guess is because there is no one solution that works for each person that has neuropathy, in my non medical opinion from what I've read.

Scientists have looked at molecules in blood of healthy people and compared them with sick people. Easy peasy. They even count them. Hence they can figure out a dosage. Eat proper food.. Bye

I believe it's part marketing, part science and part the opinion of medical professionals that adjusting within guidelines may or may not help but can't hurt. My cardiologist told me not to take over the counter vitamins and supplements because there's little control over them. My PCP put me on B12 injections for my neuropathy and it's been a lot of help.

Do you have chemo induced neuropathy or diabetic neuropathy?

Neither, I am not a diabetic and have not taken chemo. The cause of my PN has not yet been determined.

Vitamin B12 injections provides some temporary relief. I learned recently one of my three main arteries in the legs is 100% blocked.

Vitamins and minerals have recommended daily dose guidelines set by the FDA. Hopefully, doctors will test for nutrient deficiencies before randomly prescribing or suggesting vitamins and minerals. Therapeutic doses, temporary higher doses, are often recommended for those who are deficient. When your doctor prescribes a vitamin or mineral, it is wise to first ask for testing for actual deficiency because some vitamins and minerals in high levels can be toxic.