As you say you've " tried oral B12, which made no difference " it may well be that you have an absorption problem and will need injections

This is often measured by an increase in alpha wave production in the brain, which was illustrated in a study looking at L-theanines relaxation potential after 40 minutes of administration in a dose range of 50-200mg.13 A systematic review of L-theanine specifically studied for sleep declared that a range of 50-655mg could improve sleep onset latency, total sleep time, sleep efficiency, overall sleep quality, and next-day alertness.17 Neurological health (including memory and attention) Animal studies have shown that L-theanine can cross the blood-brain barrier through both oral and intravenous administration.18 19 Once there, in addition to its calming effect, L-theanine has been shown to improve recognition and recall scores in patients with mild cognitive impairment at as little as 60mg/day.20 It was also shown to improve attention and reaction time in subjects at 200mg/day on its own or combined with 160mg of caffeine.5 21 Longevity There is limited human evidence looking at L-theanine in isolation, but animal studies have shown a small increase in average lifespan and maximum lifespan with L-theanine supplementation.22 And a cohort study looking at participants with constant tea consumption status found that tea consumption effectively slowed biological aging.23 More direct human research is needed, but these results are promising

Also, GLP-1 RA seems to regulate glucose homeostasis by exhibiting glucagonostatic effects meaning suppression of glucagon by acting on pancreatic cells, which in turn reduce hepatic production of glucose (52)
Vitamin B 12 is essential for the nerve and brain health, red blood cell and DNA production, metabolism and preventing anaemia
De Rey-PailhadeMJ
2017) for protocol-specific concentrations