This research will involve two treatment groups: (children 2-5 years old) and (children 6-8 years old). The first group will include 20 children 2-5 years old. In this group the children will be randomized to either receive 0.1mg/kg of liquid oxycodone via an orogastric (OG) tube after induction but before the procedure at the beginning of general anesthesia, or 0.1mg/kg of liquid oxycodone via an OG tube after the procedure at the end of general anesthesia. Every patient will receive inhalational or intravenous induction of anesthesia as decided by the anesthesia team on the day of surgery, as is routine clinical care. An intravenous cannula (IV) will be inserted in every patient as part of their routine clinical care. No additional intravenous line will be required for this study. An arterial line will only be used if indicated by surgical procedure. As part of the study protocol, a blood sample (5 mL) will be taken from the IV and sent for genetic analysis. The genetic testing is specifically to analyze the following genotypes only: cytochrome P450 (CYP) 2D6 and 3A4, which represent the differences in cytochrome P450 metabolism of oxycodone. An orogastric tube will be placed in the stomach (placement verified by routine accepted clinical guidelines) under anesthesia as is part of standard routine clinical care to remove gastric contents. The same orogastric tube will be used for intragastric liquid oxycodone administration in a dose of 0.1 mg/kg either before the procedure (beginning of general anesthesia) or after the procedure (end of general anesthesia). This weight-adjusted dose of 0.1 mg/kg is administered as per standard clinical dosing guidelines at Boston Children's Hospital..

The second group will include 20 patients (children 6-8 years old). In this group the children will be randomize to either receive 0.1 mg/kg of liquid oxycodone per oral (PO) before the surgery in pre-op holding, or 0.1 mg/kg of liquid oxycodone PO after surgery in the Post Anesthesia Care Unit (PACU).

As part of routine clinical care, the anesthesiologist may give any of the following narcotics for additional pain management based on their clinical judgment: Fentanyl, sufentanil, and remifentanyl. For the purposes of this study, the following narcotics will not be administered: morphine and hydromorphone. Additionally, oxycodone will not be administered after the initial study dose in the operating room. These medications produce metabolites that would interfere with the pharmacokinetics being studied. Furthermore, morphine, hydromorphone and oxycodone will be withheld during the entire study period (up to 24 hours). This is not standard clinical care, but withholding morphine/hydromorphone/oxycodone will not result in any additional risk for the patient, since there are equally effective pain management options available which are standard of care. If the clinician deems that morphine, hydromorphone, or oxycodone is necessary for clinical care, it will not be withheld, and the patient will be withdrawn from the study.

A total of 11 blood samples will be drawn for the study. The first sample will be sent for genetic testing. The other 10 samples will be drawn at the following time points: sample 1: within 2 hours of post dose oxycodone, sample 2-7: within 30 to 90 min after last sample drawn and within 12 hours post dose oxycodone, sample 8-10: anytime between 12 to 24 hours post dose oxycodone (assigned as clinically indicated based on duration of surgery to minimize patient disturbance). Therefore, blood samples (1 mL each) will be drawn at 10 different time points in each patient. Blood for the study purposes will be drawn from the existing intravenous line or arterial line at the specific time points indicated when the child is under anesthesia. Postoperatively, regarding the timing of the blood draws, it is not necessary to be exact but rather to accurately record the time that the sample is taken. Thus, this will be coordinated with the drawing of routine laboratory tests if possible, in order to minimize patient discomfort.