If you've been taking metformin for more than a year or two it is probably worth getting your B12 levels checked, but my understanding is that if it is only metformin that is reducing B12 levels then a supplement should be all that's required (and if your body is tolerating metformin OK then presumably the benefits outweigh a slight reduction in B12 that can be corrected in this way). Major B12 deficiency can cause serious problems with nerve damage -- but mine was not related to my diabetes or its treatment, and even then at least some of the damage can be offset by B12 injections. So don't panic, metformin-takers -- but just monitor it.