Purpose. To assess the extra information about patients' pre-admission medication that second drug interviews and general practitioners' (GP) drug lists add to routine medication histories. Methods. The medication histories of 81 patients from two medicine wards were compared with information collected from second interviews and GP drug lists. A panel of clinicians assessed the potential clinical impact of any discrepancies. Results. In nine cases (11%CI: 6-20%) the medication history, the second interview and the GP lists were identical. The interviews revealed extra information about 54 prescription drugs and 72 over-the-counter drugs (OTCs) in 56 (69%CI: 58-78%) cases. The GP lists added 74 prescription drugs and 26 OTCs to the medication histories in 57 (70%CI: 60-79%) cases. Discrepancies were not associated with age, gender or number of drugs in medication history. In 18% (CI: 10-30%) of the cases, the specialist panel ascribed potential clinical problems to missing drug information. Conclusions. Second interviews and GP lists reveal extra information about pre-admission medication in two-thirds of cases. Both procedures should be routinely performed to compile a more comprehensive basis for drug prescribing. This approach may mend the course of roughly one in five patients.