JAMA 2018;320, 13191320, doi: Tchkonia, T., Zhu, Y., van Deursen, J., Campisi, J
In a multicentre, examiner-blind, clinical and neurophysiological study lasting 4 months, 120 hands from 82 patients with mild-to-moderate carpal tunnel syndrome were examined at baseline and 10, 60, and 120 days after treatment with 500 mg of acetyl-l-carnitine twice daily for 10 days intramuscularly followed by oral administration at 500 mg twice daily for 4 months
So knowing what's in the bloodstream is only indicating how much B12 has not been absorbed or used by the body yet
If you know you'll be away from home on your usual injection day, plan ahead by either taking your medication with you in a cooler with ice packs or adjusting your schedule with your provider's guidance before the disruption occurs