I.C.I. 35868 - The effect of a change of formulation on the incidence of pain after intravenous injection.

I.C.I. 35868 is a promising new intravenous induction agent, but intravenous injection of the initial 2% formulation frequently caused pain. The solvent vehicle of the 2% formulation, which contained Cremophor E.L. and ethanol, was therefore investigated to see if this could be the cause of the pain. Twenty volunteers received both 5 ml isotonic saline and 5 ml solvent vehicle intravenously. The injection of saline never caused pain, but the solvent caused mild or moderate pain in 9 patients. I.C.I. 35868 was therefore reformulated in a 1% solution, excluding ethanol from the solvent. When intravenous injection of I.C.I. 35868 1% was compared to methohexitone 1% in 24 patients, both agents caused pain. Four out of 12 patients reveiving I.C.I. 35868 1% felt pain, with a total pain "score" of 6, but 6 out of 12 patients who received methohexitone 1%, felt pain, with a total pain "score" of 12. In no instance was intravenous injection followed by persistant pain, or evidence of thrombosis or phlebitis.