"A number of drugs antagonize folate by mechanisms that are poorly understood but are thought to involve an effect on absorption of the vitamin by the intestine. In this category are the anticonvulsants phenytoin, primidone, and phenobarbital. Megaloblastic anemia induced by these agents is mild."
Now there are several references to B12 soprtion problems from phenytoin, however, with a person with a normal dietary history prior to starting phenytoin, they should have an adequate supply of B12, as the average person has a 6-9 year supply of b12 in stores. So in cases where the phenytoin has been used for a short term, the answer should be folate, for long term cases, probably both.