The reduction in propofol-induced pain on injection caused by the addition of lignocaine results mainly from a drop in pH, which reduces the concentration of propofol in the aqueous phase of the emulsion. It is not an effect of the local anaesthetic per se. Propofol emulsion mixed with lignocaine destabilizes within hours. We mixed 10 parts of propofol 1% emulsion with one part of 0.0064 m HCl or 0.013 m HCl, respectively. These mixtures were stored for 3 months and compared with a freshly prepared solution of propofol 1% emulsion and saline, in the same proportion, regarding their ability to induce anaesthesia in the rat. There was no significant difference in the amount of propofol required to induce anaesthesia, nor was there any difference in recovery time between the three groups.
