weekly b12 injections Boost Your Energy with Shots in Catonsville MD B12 Injection Benefits: Energy, Mood,
Description
What follows is what we know from that review and from the published clinical record

In the UK, hydroxocobalamin is the preferred licensed form for intramuscular injection
Adverse Reactions/Side Effects Generalized: Anaphylactic shock and death have been reported with administration of parenteral vitamin B 12 (see WARNINGS)

That is intentional: the two vial sizes reflect two different documented protocols and two different concentration setups, and it is a reminder never to eyeball a dose across vial sizes