Morphine and tramadol are the opioid analgesic drugs acting via activation of μ‑opioid receptors. It is important to understand which mechanism (synergistic or additive anti‑nociceptive activity) induced potent anti‑nociceptive effect by co‑administration of morphine and tramadol. Identification of new strategies that can potentiate analgesic effects of opioids will be good therapeutic approaches for pain relief. To this aim, male mice were cannulated in the left ventricle by a stereotaxic instrument. A tail‑flick test was used to record the pain threshold. The results revealed that intracerebroventricularly injection of morphine induced an anti‑nociceptive effect in non‑sensitized and morphine‑sensitized mice. We found that infusion of tramadol produced an anti‑nociceptive response in non‑sensitized mice, whereas tramadol in doses of 0.5 and 1 μg/mouse induced analgesia in morphine‑sensitized mice. Co‑injection of a non‑effective dose of tramadol or morphine (0.25 μg/mouse) with different doses of morphine or tramadol (0.25, 0.5, and 1 μg/mouse) respectively potentiated the analgesic effect of the previous drug. An isobolographic analysis of data was performed, indicating a synergistic interaction between morphine and tramadol in non‑sensitized and morphine‑sensitized mice. Our data indicated that both morphine and tramadol elicit more anti‑nociceptive response in morphine sensitized mice; there is a synergistic effect between morphine and tramadol upon induction of analgesic effect in non‑sensitized and morphine‑sensitized mice.