Most B Complexes are very low in B-12. There are a couple different kinds of B-Complex. B-50 Complex and B-100 Complex are just B vitamins at 50 or 100 mgs for most and 50 or 100 mcgs for B-12 and some others. 50 or 100 mcgs of B-12 is very low. Most also use cyanocobalamin vs methylcobalamin. Methyl is better.

Super B Complex forms are B vitamins with other vitamins and are also usually low in B-12. They often dose B-12 based on the RDA levels. The RDA levels for B's are almost useless.

Most need 1000 to 2500 mcgs of B-12.

I don't get any tiredness for gabapentin. I was taking 1200 mgs for the first few years but reduced to 900 and now just 600 mgs before bed.

Which B Complex brand are you taking ?