The NHS delivers B12 injections through a nurse or doctor for exactly this reason
Have Severe Chronic Conditions Requiring Medical Supervision Beyond Supplements: If you are managing serious chronic health issues, your care should be directed by qualified healthcare providers rather than self-directed supplementation
Proton pump inhibitors, like omeprazole, can also impair vitamin B12 absorption by reducing stomach acid production
Symptoms may include itching, rash, swelling, or even difficulty breathing in severe cases
If switching is necessary, allow a 35 day washout period to clear residual plasma levels, then treat the new route as a fresh baseline
The headaches seem to follow a different pattern (from what I remember) and tend to lessen over time - I had problems with headaches when I started using methyl but (nasal rather than injection) but it went away after a week or so