Wordliczek 2002.

Methods	Study design: parallel‐group randomized controlled trial Sample size: 90 Country: Poland Setting: secondary care hospital Dates conducted: not reported Postoperative opioid used and delivery: PCA tramadol Pain score collection: not reported Concurrent postoperative analgesics: none reported
Participants	Inclusion criteria Scheduled for elective colon surgery (hemicolectomy) Exclusion criteria Severe hepatic, renal, cardiovascular, or psychological disorders Unable to rate the VAS score Unable to use the PCA pump
Interventions	Group I (30 participants): 100 mg IV tramadol 15 minutes before induction Group II (30 participants): 100 mg IV tramadol after peritoneal closure Group Control (30 participants): 100 mg IV tramadol immediately after operation
Outcomes	Tramadol consumption (mg in early postoperative period) Postoperative pain (VAS on initial activation of PCA device, 2, 4, 8, 12 hours postoperatively, and the morning of the day following surgery) Time to analgesia (minutes) Drowsiness/sedation (yes/no postoperatively) Nausea and vomiting (yes/no postoperatively)
Notes	Funding: supported by grant WL/241/KL/L from Collegium Medicum Jagiellonian University, Kraków, Poland Declarations of interest: none reported Authors contacted: we contacted authors for further information but received no response. Other: only results from Group I (preventive) and control group compared. Sedation not analysed as dichotomous outcome. Time to analgesic request standard deviations estimated as unclear from graph.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	No details. Quote: "participants were randomly allocated into three groups"
Allocation concealment (selection bias)	Unclear risk	No mention
Blinding of participants and personnel (performance bias) All outcomes	High risk	Interventions given at different times. Quote: "30 patients (I group) were administered 100 mg of tramadol iv 15 minutes before induction of general anesthesia. The patients of II group were administered 100 mg of tramadol iv immediately after peritoneal closure. In order to determine the influence of the inhibition of phase II (inflammatory response in the postoperative period) on nociceptive stimulation, patients in control group (30 patients) received 100 mg of tramadol iv immediately after operation"
Blinding of outcome assessment (detection bias) All outcomes	High risk	Likely unblinded based on above information
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	No mention of any dropouts
Selective reporting (reporting bias)	Unclear risk	No protocol or trial registration
Other bias	Low risk	Similar baseline characteristics and no reported conflicts of interest