In the treatment of undiagnosed megaloblastic anemia, vitamin B12 and folic acid should be given together because :
Vitamin B12 is required for conversion of methionine to homocysteine and for the formation of succinyl CoA from methylmalonyl CoA.
Deficiency of vitamin B12 results in megaloblastic anemia, GI manifestations and neurological abnormalities (due to demyelination).
Folic acid alone will correct the symptoms of megaloblastic anemia but it does not prevent neurological abnormali­ties, which continue to proceed.
Neurological abnormalities manifest initially in the form of loss of posterior column sensations (vibration, proprioception etc.), but later on, can result in subacute combined degeneration of the spinal cord.
Vitamin B12  acts as a cofactor for dihydrofolate reductase
Folic acid alone causes improvement of anemic symptoms but neurological dysfunction continues.
Vitamin B12, deficiency may result in methylfolate trap
Folic acid is required for conversion of methylmalonyl CoA to succinyl CoA.