Abdulla 2012a.

Methods	Double‐blind, placebo and active‐controlled, multiple dose, over 24 h First dose administered 15 min prior to extubation
Participants	Type of surgery: thyroidectomy Paracetamol group Entered/completing: 30/30 Age (mean, SD): 44.5 ± 15.1 Sex (male, %): 16.7% Placebo Entered/completing: 30/30 Age (mean, SD): 47.9 ± 11.8 Sex (male, %): 10.0% Parecoxib Entered/completing: 30/30 Age (mean, SD): 48.3 ± 14.2 Sex (male, %): 23.3% Metamizole Entered/completing: 30/30 Age (mean, SD): 43.8 ± 13.7 Sex (male, %): 16.7%
Interventions	1 g paracetamol in 100 ml normal saline over 15 min Placebo, parecoxib 40 mg, or metamizole 1 g: all in 100 ml NS over 15 min
Outcomes	Primary: accumulated opioid consumption (piritramide via PCA) Secondary: Pain intensity (VAS) Pain relief (VRS) Patient satisfaction (VRS)
Source of funding	Not mentioned
Were treatment groups comparable at baseline?	Yes
Details of preoperative pain	Participants with chronic pain were excluded
Notes	—
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computer‐generated randomization (http://www.randomization.com)
Allocation concealment (selection bias)	Low risk	Sealed, opaque envelope, only opened in emergency
Blinding (performance bias and detection bias) All outcomes	Low risk	“The study solutions were clear so that they could not be recognized by the anesthesiologists collecting the data and were prepared by one of the researchers who was not involved in the intraoperative and postoperative treatment of these patients”.
Incomplete outcome data (attrition bias) All outcomes	Low risk	No dropouts or protocol violations – complete data set obtained for all 4 groups
Selective reporting (reporting bias)	Unclear risk	All outcomes from Methods section reported in Results section, but side effects not monitored adequately
Size	High risk	Fewer than 50 participants per arm of the study (30 paracetamol, 30 placebo, 30 parecoxib, 30 metamizole)