It’s worth noting that most doctors giving B12 shots are using the specific form cyanocobalamin, which is the cheap synthetic version that has cyanide in it. They typically give mega-doses (10-25 mg) to help those with B12 deficiency symptoms.
However, this is not necessarily required. Cyanocobalamin has to go through a complex conversion process within the body in order to become active and many people cannot do these conversions well, so much of the dose goes unused (even with a shot).
For some overloading their system with a mega-dose shot of B12 can actually make them pretty sick because because they can’t clear or convert that form of B12 very well. This can be due to MTR or MTRR genetic mutations which help the body convert B12 into its active forms for use at the enzymatic and cellular level. When these conversions don't happen, then the body's blood stream is overloaded with a form of B12 it can't use or convert and in many cases can't clear very quickly either (hence you feel very bad afterwards).
If you've had a BAD experience with B12 shots, it's most likely that you do not tolerate or convert cyanocobalamin well (see if you can verify what form of B12 was in the shot that made you react poorly - and then stay away from that specific form if you can).
A very few naturopaths or functional medicine doctors may be using methylcobalamin in an IV or shot scenario which is considered one of the most active forms of B12, and it works well for many, however, it’s typically the least well-tolerated of the 3 active forms.
The B12 form that most people tolerate best is hydroxocobalamin. Hydroxocobalamin is a rare and somewhat expensive, but also ‘active’ form of B12 that’s very well-tolerated by most people. This is the form we at Methyl-Life™ recommend (along with many naturopaths and nutritionists who understand B12) because it has the best overall effect for its users. Not only is it an active form itself, but it also converts into two other active forms within the body (methylcobalamin and adenosylcobalamin).
And believe it or not, the studies tell us that sublingual B12 is just as effective as intramuscular B12 shots, so find yourself a good source of hydroxocobalamin (and put it under your tongue to dissolve). Methyl-Life™ carries an affordable, vegan version of hydroxocobalamin in a tasty sublingual form.
And if you're looking for a full-spectrum B12 product that contains all three active forms of B12, then check out Methyl-Life's™ B12 Complete product to learn more. It contains a therapeutic dose of hydroxocobalamin as well as methylcobalamin and adenosylcobalamin to bioactively meet all of your body's B12 needs.