1.5 yr of neuro sx. mri, emg, neuro workup clear. slightly low b12 at 384 pg. pcp said i could do daily cyano shots to see if sx clear. some have, others persist. could 1 yr of daily b12 shots be harmful? worried it could somehow increase cancer risk?

B12 per se is not a cancer risk. You do not need daily injections to make up any B12 deficit, once monthly or even once in three month injection will provide sufficient B12 to correct deficiency. Talk to your doctor if sublingual B12 may be adequate to replace injections. For info: https://pubmed.ncbi.nlm.nih.gov/30632091/

Unlikely your B-12 level is low enough yet to cause clinical symptoms, but would not discourage supplementation, and you solely would need once monthly injections, at most. You might further explore your cancer fears, but B-12 does not cause cancer.