While the antibiotics used to knockdown intestinal bacteria are non-absorbable and are excreted in the feces, the possibility remained that alterations in gut bacteria could alter the expression of hepatic enzymes which are responsible for metabolism of cocaine – thereby altering the effective dose the animal would receive in a weight-based dosing model. To test this, we measured serum levels of benzoylecygonine, the primary metabolite of cocaine, after cocaine injection. Levels were monitored in a time course after a high dose (20 mg/kg) of cocaine (Fig. 2a). A two-way ANOVA analysis of these results demonstrated the expected effect of time (Time: F(2,19) = 766.3, p < 0.0001), but there was no effect of antibiotic treatment (Treatment: F(1,19) = 2.27, p = 0.15) or any time x treatment interactions (Interaction: F(2,19) = 1.61, p = 0.23). Given that our primary behavioral effects were seen with a dose of 5 mg/kg, metabolism of this dose at 30 minutes post-injection was tested as well (Fig. 2b). As with the higher dose, there was no effect of antibiotic treatment on the metabolism of cocaine
