I gotta interject here and say either you aren't doing as much nitrous as you claim or something else is off.
For me after about 0.5L per day, after 6 weeks I began having clear neuropathy (in my 30s). My b12 levels were low for months afterwards, regardless of taking aggressive doses of methylated b12.
Nitrous isn't some poisonous gas by any means, most of our air is some combination of nitrogen + oxygen... but its also not good for you and not harmless by any means. Nitrous addiction typically leads to permanent damage compared to a lot of other drugs.