I have been taking LDN for a few months now -and it has helped quite a bit with brain fog /concentration/ motivation issues. I started at 1mg the first month and bumped to 2mg at month 2. I still have occasional waves of issues, but less frequent.. Here is a link to a legit study on the efficacy of LDN on Long Covid patients.. My doctor used this to feel more comfortable prescribing it for this purpose. I brought this to here attention, as i had trouble getting her to research any remedies. She said - "ya.. that sounds good.. lets try it. " Oy - not helpful or reassuring - but at least she was able to prescribe it for me.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9250701/
FYI - I am also taking Duloxotene for 6 months -that has also helped. Not sure if I should stop that now that LDN seem to be helping more..