Abstract This study was designed to determine whether the acquisition cost of valproic acid, which is lower per unit than that of divalproex sodium, actually translates into overall savings when additional costs for gastrointestinal (GI) medications, procedures, and consultations related to side effects are included. The research was conducted at Western State Hospital, Staunton, Virginia. It was prompted by a policy change in the mid-1990s requiring that all patients treated with divalproex sodium be switched to valproic acid because of its lower acquisition cost ($0.90/unit vs $0.33/unit). Data were collected for all patients hospitalized between July 13, 1992, and March 14, 1997. These data included doses and total costs of both drugs, GI medications prescribed, procedures performed, and relevant consultations held, as well as the reasons and costs for these medications, procedures, and consultations. Of the 36 patients identified, 15 had bipolar disorder, 11 had schizoaffective disorder, 7 had schizophrenia, and 3 had seizure disorder (1 with delirium secondary to the seizure disorder). Nineteen patients started with valproic acid and switched to divalproex sodium; 8 started with divalproex sodium and switched to valproic acid; and 9 started with divalproex sodium, switched to valproic acid, then switched back to divalproex sodium. Twenty-seven patients were unable to tolerate valproic acid because of GI symptoms and were switched (or switched back) to divalproex sodium. All 27 of these patients remained on the drug. Only 7 of the 36 patients were successfully treated with valproic acid and had no need for GI medications, procedures, or consultations. In 28 patients treated with valproic acid, side effects resolved or were alleviated when they were switched to divalproex sodium. Only 8 patients completed their hospital stays taking valproic acid. The 8 patients who started on divalproex sodium and were discharged on valproic acid had mean treatment-related costs of $106.82/patient. This is compared with $18.38/ patient for the 28 patients who were treated with valproic acid but discharged on divalproex sodium. In conclusion, although the acquisition costs of valproic acid were lower than those of divalproex sodium, treatment-related costs were higher, making valproic acid the more expensive drug to administer overall.