Powerful behavioral interactions between methamphetamine and morphine

► Interactions between methamphetamine (MA) and morphine (MOR) were examined in rats. ► MA caused a stimulant effect while MOR produced an inverted U-shaped dose response. ► The combination of MA and MOR produced effects greater than either drug alone. ► The greater than additive effect was dependent on the doses and behaviors examined. ► The results demonstrate complex behavioral interactions between MA and MOR.