Ethanolic extract of the leaf of Calotropis procera
was investigated for its anti-inflammatory and analgesic activities. The extract was evaluated using formalin-induced paw lick, carragenaan-induced paw oedema in Wistar rats, acetic acid-induced writhing and tail flick tests in mice. Each experiment consisted of thirty animals randomly, but equally divided into groups of 100mg/kg, 200mg/kg or 400mg/kg body weight (b.w) of extract, Indomethacin (10 mg/kg b.w) or aspirin (15mg/kg b.w) pre-treated animals and a control group administered with distilled water (10ml/kg b.w). The administration of the extract was repeated for formalin-induced paw lick and acetic-acid induced writhing models in the presence of an opioid antagonist, naloxone. The data were analyzed using one way ANOVA and difference of means were considered significant at p <0.05. The ethanolic extract exhibited potent anti-inflammatory or analgesic effect in this study. Inhibition of formation of paw oedema by the extract (100mg/kg b.w) was significantly higher than for Indomethacin. Itching was significantly reduced in rats administered with extract in the early phase of formalin response, and was comparable to Indomethacin (10mg/kg b.w). 100 mg/kg body weight of the extract also inhibited the writhing movement comparably with aspirin (15mg/kg b.w). Same pattern was also observed with tail flick model in mice. The study showed that the mechanism of action of the analgesic or anti-inflammatory action of the leaf extract is mediated both centrally and peripherally. The analgesic or anti-inflammatory effect of the extract was not attenuated by opioid antagonist, naloxone, thus ruling out the involvement of opioid receptors in the central mechanism of action of the extract. It was therefore concluded that these activities are mediated via interaction with other nociceptive pathways.