I wanted to just share this in case anyone else has similar issues with reacting badly to some antibiotics (particularly Trimethoprim or Bactrim) after developing a B12 deficiency.
I think I might've figured out why I am responding so badly to antibiotics I've tried over the past few months, that I used to have no problem with at all in the past.
It seemed to me that it had to be something fundamental, and based on my history, it could easily be because of a B12 deficiency. And from my communication with others, it appears that people who have pernicious anemia or B12 deficiency from diet, or Nitrous Oxide, have less tolerance to antibiotics they also took with no problem before becoming deficient.
Obviously B12 is crucial for nerves (as is folate), DNA synthesis and cell metabolism. It also keep methylmalonic acid and homocysteine low, which both can cause toxic effects in high amounts. If there nerve cells are already experiencing a high level of oxidative stress and damage, and nerves become hyperexcitable, then adding antibiotics on top of that could push things over the edge where I'd develop worse symptoms.
Doxycycline is more confusing to me, but that has important effects on cells mitochondria by inhibiting protein synthesis. And Trimethoprim has effects on folate metabolism, through its inhibition of dihydrofolate reductase.
Interestingly, Trimethoprim can lower folate levels quite rapidly, within days. And a Vitamin B12 deficiency can cause a functional folate deficiency, as explained by the 'methyl trap hypothesis' - this is where cobalamin (B12) deficiency causes reduced methionine synthase, resulting in a 'functional' deficiency by increasing the proportion of folate as the 5-methyl-derivate.
Before I figured this out from my research, I knew that B12 deficiency could increase the risk of toxic side effects from several chemotherapy drugs, which massively increased the risk of peripheral neuropathy. For example:
"A total of 12 patients had folate deficiency, and 10 of those suffered from severe toxicity (grade 3 or more)"
"Pemetrexed (Alimta®) is a novel multi-target antifolate that inhibits several enzymes involved in DNA synthesis such as thymidylate synthase (TS), dihydrofolate reductase (DHFR), and glycinamide ribonucleotide formyltransferase (GARFT). Early studies showed a severe toxicity profile, but further studies performed in the US yielded a lower grade of toxicity. The reason for that was assumed to be folate supplementation in flour (7). Therefore Alimta is given together with folate and vitamin B12 supplementation." - PMC2852767/
That's just one study, but there are others, including case studies, where patients who were found to have a B12 deficiency developed grade 3 neuropathy, and had to stop the chemo drug. Then once the B12 deficiency was corrected and their neuropathy improved to grade 1, these patients did not progress beyond grade 1. Therefore, B12, as a fundamental vitamin to nerve function and regeneration, can alter the tolerability of neurotoxic drugs.
Based on some symptoms I had over 2022, it's probably, perhaps highly likely I had a B12 deficiency. But I cannot know for sure because I had already begun supplementing a small amount of B12 a month prior to the infection, albeit a small amount of 160 mcg, which is far too small to correct neurological functions and even haematological abnormalities, which requires at least 500 mcg. And finally, 1000-2000 mcg of methylcobalamin is required for optimal reductions in homocysteine and methylmalonic acid.
Anyway, that the hypothesis at the moment. How plausible does this sound???
B12 and folate relationship.
A clinical picture of a B12 deficiency and PA explained.........
Role of B12 and Folate in the Conversion of Homocysteine to Methionine 
Testing for Functional B-12 Deficiency
