I've been suffering with low B12, PVC's and AF for a number of years.
I decided to switch to self injection around 3 years ago, due to the local GP's not offering anything more than 4 a year after loading.
Had a number of symptoms, some still remain.
I also switched to more regular self injections, Subcutaneously over IM due to both pain and nausea post injection. I now do 3-4 injections of around 0.25ml a month over 1ml monthly IM. For me this seemed to give less side effects.
More recently I've discovered, after severe gallbladder problems and an endoscopy that I have a dairy allergy and struggle to digest meat. Bizarrely after discontinuing both meat and particularly dairy my stomach issues have completely disappeared. Even more bizarrely after discontinuing all forms of dairy all heart arrythmia (PVC's and AF) have also completely discontinued.
For anyone suffering similar effects it may be worth experimenting with dramatic diet changes. Sadly for me, this meant giving up Coffee (caffeine), Chocolate, butter, cream, milk and all other dairy. Meat, particularly red meat seems to exasperate stomach issues, I can tolerate some fish, small amounts of Chicken but very little red meat. All of which add to the lack of B12 but the injections seem to mitigate this.
It's extremely refreshing not to feel at deaths door several times a day, hope this may help others.