It is intended to provide an oral preparation which is usable in diagnosing the presence or absence of an abnormality in the capacity to metabolize pyridine, a pyrimidine metabolic speed and so on at a high accuracy with little difference from individual to individual. An isotope-labeled compound and/or a pyrimidine metabolite compound (a) are mixed with a sugar and/or a sugar alcohol (b) and milled. By using the powdery material thus obtained, an oral preparation is produced.
