Morphine and fentanyl interactions with thiopental in relation to movement response to noxious stimulation.

The effects of morphine-thiopental and fentanyl-thiopental combinations on the movement response caused by tail clamping were studied in rats. Doses that prevented movement response when the agents were given singly and when the agents were given in combination were determined by a probit procedure and compared with isobolographic analysis. With doses of the above agents sufficient to block the movement response to tail clamping (ED50 values: 4.7 (3.3-6.6) mg/kg intravenously for morphine; 8.3 (5.8-11.3) micrograms/kg intravenously for fentanyl; and 18.8 (17.9-19.7) mg/kg intravenously for thiopental) both fentanyl and, to a lesser extent, morphine have a less than additive or an antagonistic interaction with thiopental. This antagonism is a relative one that does not increase the requirement for one agent upon the addition of another agent.