Pharmacological effects of a combination of butorphanol and acetaminophen

Isobolographic analysis was used to study in mice the combined oral analgetic action of butorphanol, a new centrally acting analgetic, and acetaminophen, an antipyretic analgetic. Combinations of butorphanol-acetaminophen exhibited analgetic effects which were greater than were expected from just an additive analgetic action of each drug. Consequently, a significant synergistic effect occurred after the simultaneous oral administration of butorphanol and acetaminophen. Furthermore, oral administration of butorphanol-acetaminophen mixture to conscious dogs at approximately 20 times the recommended human dose resulted in little or no effect on aortic blood pressure and arterial blood pH,pCO2 and pO2. Heart rate was slightly reduced and this resulted in a small increase in the PR interval and a prolongation of the QT interval of the lead II surface electrocardiogram.