The aim of this paper is to examine the prescribing patterns and cost of various formulations of metronidazole in a hospital setting over a 3-month period. Oral metronidazole has high bioavailability (98.9%) with peak plasma concentrations averaged at 2.3 h after dosing. Despite the high bioavailability of oral metronidazole, many patients continue to receive metronidazole intravenously when they are suitable for oral preparation. An audit of 120 consecutive patients prescribed metronidazole was conducted at the Liverpool Hospital, NSW, from March to July 2005. There were 65 men and 55 women (age 18-93). Of the 120 patients, 16 were on oral, 1 on rectal and 103 were on intravenous metronidazole. Treatment was initiated based on clinical diagnoses. Potential pathogens were subsequently identified on only 21 occasions. The use of metronidazole as an oral preparation was contraindicated in 27 patients (22.5%) who were nil-by-mouth. Of these, rectally administered metronidazole was contraindicated in only eight patients. The average course of intravenous metronidazole was 8.0 +/- 9.7 days (mean +/- SD). The total number of intravenous metronidazole treatment days was 824. Oral metronidazole would have been possible in 618 out of the 824 days. The estimated cost to administer each dose of oral, suppository and intravenous forms of metronidazole is $A0.11, $A1.34 and $A6.09 respectively. Thus, substantial savings could be achieved if oral metronidazole were to be administered whenever possible. The early use of oral or rectal metronidazole should be encouraged when there are no clinical contraindications.