My GP would only inject me with B12 every 12 weeks, despite having neuro symptoms (depression, headaches, tremors, long "pauses" in being able to think), which NICE guidelines suggest should result in 8 week injections. Around 8 weeks I'd have symptoms flood back. My GP thinks B12 is hokum placebo but that's completely incongruous to everything I've read.
Not diet related for me, but other members of my family appear also to not absorb B12 through their gut. However, being sublingual I thought I'd try these and after a couple of weeks I'm certain they're working. Your mileage may vary I'm sure, depending on whether you're B12 deficient and the reason why.
Postage was very expensive, so I'll probably be looking to source in larger quantities.