Drug Interactions Between Paracetamol and Setrons in Pain Management

Tonsillectomy is a common operation performed in children that is associated with marked pain and a high incidence of nausea and vomiting. Therefore, national and international guidelines have recommended the use of a setron as a prevention to nausea and vomiting and also the systematic administration of paracetamol to help in the control of postoperative pain. However, a potential interaction between setrons and paracetamol has been reported in different animal studies and in human volunteers although its existence has never been found in the clinic. Indeed, several hypotheses are proposed to explain the mechanism of action of paracetamol in the treatment of pain. However, a recent one involves the endocannabinoid system and spinal serotonin receptors. Serotonin receptors are also involved in the mechanism of action of antiemetics such as setrons. Indeed, these drugs are serotonin antagonists. Therefore, the investigators hypothesized that the concomitant administration of paracetamol and ondansetron leads to an interaction that will decrease the analgesic effect of paracetamol.

Patients aged 2-7 years old and scheduled for a tonsillectomy will be recruited. They will all receive intraoperatively intravenous paracetamol together with either ondansetron or droperidol. Pain scores using the CHEOPS scale will be recorded for 24 hours. Furthermore, patients will receive i.v. morphine during the operation and in the recovery room if necessary and a non-steroidal anti-inflammatory drug for postoperative pain; on the ward, oral codeine will be administered when needed. Pain scores will be recorded regularly for up to 24 hours together with opioid consumption and the incidence of nausea and vomiting in the same period. Any adverse event will also be recorded.

Inclusion criteria

• tonsillectomy in a child 2-7 years old who is hospitalized for at least 24 hours ;
• informed consent from one parent at least

Exclusion criteria

• hospital stay of less than 24 hours ;
• patient already on pain medication ;
• allergic patient with a contra-indication to one of the study drugs.