Injectable and oral have been studied recently and found to yield the same results. So why use injectable? Don't make it more expensive and complicated for nothing better.

Well for very low critically ill patients injectable is often started at 1000mcg/day for about 7 days in a row. Absorbed from that is about 145mcg.

typically injections for others are once a month. This typically does not work well, and only provides a yo-yo response, of feeling better for a few days, then declining again until the next shot.

Here is a link with a study comparing them.. oral and injectable.
One has to take oral on an empty stomach to get the comparable returns as microgram doses can be bound to food present and reduce the passive absorption.

http://www.ncbi.nlm.nih.gov/pmc/arti...able/T1/#TF1-4

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532799/

http://www.aafp.org/afp/2003/0301/p979.html

Trust me, most people will have levels of 1000 or more after 3 months of oral. Many of our PNers have done this here over the years. You might even get higher levels, like I did of 1999. Most labs only calibrate to levels of 2000.
Oral costs pennies a day... can't beat that price either.