Nalmefene is an orally available opioid receptor antagonist that has been shown to suppress appetite in humans, but its effects on chronic food intake and body weight remain unclear. Here, we report that chronic (21-day) oral administration of nalmefene at 2 or 10 mg/kg/day in diet-induced obese (DIO) mice led to significant increases (9–11%) in cumulative food intake. Mice in the nalmefene-treated groups also gained body weight at a rate faster than the control. Body composition analysis showed that the extra body weight gains in the treated animals were mostly due to increased fat accumulation. Since acute nalmefene treatment showed a trend toward a decrease rather than an increase in food intake, it is possible that the orexigenic effect of chronic oral administration of nalmefene was caused by pharmacologically active metabolites rather than the drug itself. Our results argue against the potential use of nalmefene for treating human obesity.