Britzi, M., Gross, M., Lavy, E., Soback, S., Steinman, A. Bioavailability and pharmacokinetics of metronidazole in fed and fasted horses.J. vet. Pharmacol. Therap.33, 511–514.Metronidazole (1‐[2‐hydroxyethyl]‐2‐methyl‐5‐nitroimidazole) is a bactericidal antimicrobial agent used for treatment of infectious diseases caused by anaerobic bacteria and protozoa. Pharmacokinetics of metronidazole following its administration to horses was previously described (Sweeneyet al., 1986;Baggotet al., 1988;Spechtet al., 1992;Steinmanet al., 2000). The bioavailability (F) was 85% (ranging from 57% to 105%) and the time to reach maximum serum concentration (tmax) was 1–2 h after oral dose at 25 mg/kg body weight (Sweeneyet al., 1986).Baggotet al.(1988)found thatFwas 74.5% (ranging from 58.4% to 91.5%) andtmaxwas 1.5 h after oral dose at 20 mg/kg body weight.Spechtet al.(1992)reported thatFwas 97% (ranging from 79% to 111%) andtmaxwas 40 min after oral dose at 15 mg/kg body weight. In an earlier study by our groupFwas 74% andtmaxwas 65 min after oral dose at 20 mg/kg body weight (Steinmanet al., 2000). These individual variations inFmight be partially explained by the effect of feed, among other factors, mainly on metronidazole absorption. Interactions between food and drugs may reduce or increase the drug effect. The majority of clinically relevant food–drug interactions are caused by food‐induced changes on the bioavailability of the drug (Schmidt & Dalhoff, 2002). In dogs, absorption of metronidazole is enhanced when given with food, but delayed in humans (Plumb, 1995). Although, metronidazole is used commonly to treat various clinical conditions in horses with relatively little adverse effects (Sweeneyet al., 1991), narrow margin of safety was suggested because histological evidence of peripheral neurotoxicity and hepatotoxicity were noted in horses treated with doses as low as 30 mg/kg body weight every 12 h orally for 30 days (Whiteet al., 1996). For drugs with a narrow therapeutic index, even small changes in dose–response effects can have significant consequences (Schmidt & Dalhoff, 2002).