You can have methyl compounded into injections. The autism community does that. Use a compounding pharmacy.

But I don't think injections are the way to go long term. The body does not handle bolus vitamins well. It wants to excrete them.
B12 is carried into the CSF and is also stored in the liver also.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC488275/

For about 100 dollars you can have a DNA MTHFR test to show if you have trouble methylating folic acid and B12. There is a special folate called l-methylfolate that bypasses this genetic error which occurs in about 1 in 10 people. It is in the RX vitamin Metanx and also OTC as MetaFolin by Solgar in 800mcg dose. There is a discussion right now on our forum here about it and the failure of a new product posing as an equivalent generic. I have more details there.
http://neurotalk.psychcentral.com/thread114302.html
It is also available RX in a very very high dose of 7.5mg as Deplin for mental depression. Regular folic acid has to be methylated in the body before it becomes useful and active.
If one has the gene polymorphism failure, regular folic acid does nothing.

On that thread I discuss the problems of TOO MUCH folate as well.

Many of us here do very well with once a day oral B12. The trick is to take enough, and to take on an empty stomach.