Pharmacokinetics of oral diclofenac and acetaminophen in children after surgery

Background: Our aim was to study the pharmacokinetics and pain scores following administration of single oral doses of either diclofenac or high-dose acetaminophen (paracetamol). Methods: In the morning, the day after tonsillectomy, children 5-15 years of age were randomized in a double-blind manner to receive either diclofenac 1-2 mg.kg(-1) (n = 11) or acetaminophen 22.5 mg.kg(-1) (n = 10). Postoperative pain was assessed by self-report and blood samples were drawn every 30 min for 4 h after medication. Results: Large interindividual differences in maximum plasma diclofenac concentrations (C-max) were found. Mean C-max was 2.4 +/- 1.3 mug.ml(-1) and mean t(max) was 2 +/- 0.5 h. No significant reduction in pain score with diclofenac was seen at any of the assessments during the study period. Eight of 10 children achieved C-max of acetaminophen within the 10-20 mug.ml(-1) antipyretic range. Mean t(abs) was 0.7 +/- 0.3 h and mean C-max and t(max) were 12.7 +/- 3.8 mug.ml(-1) and 1.4 +/- 0.5 h, respectively. No significant reduction in pain score with acetaminophen was seen at any of the assessments during the study period. Conclusions: The achieved concentrations of diclofenac and acetaminophen were not able to significantly reduce the children's pain score during the 5 h postingestion study period. Analgesic plasma acetaminophen concentrations may be higher than those required for antipyresis.