Thiopental sodium and midazolam are frequently used to induce anesthesia. It has been known that barbiturates enhance the binding of benzodiazepines to the benzodiazepine receptors and also enhance the anesthetic effect of benzodiazepines. As a results, the combined effect of a barbiturate and a benzodiazepine should be more than the sum of the separate effects of the two drugs. The purpose of this study was to evaluate hypnotic interaction between the two drugs during induction period.

The effect of midazolam on the induction dose-response curve for thiopental sodium was studied in unpremedicated 240 ASA physical status I and II female patients. As an endpoint of hypnosis, ability to open eyes on command was used. Dose-response curves for thiopental sodium, midazolam, and their combination were determined with a probit procedure and compared with a isobolographic analysis.

For hypnosis, significant(P<0.05) synergistic interaction was found between the two drugs. The dose of thiopental sodium required to produce hypnosis was reduced by 64% in the presence of equi-effective dose of midazolam.

The interaction between thiopental sodium and midazolam for hypnosis is synergistic.