1. Key definitions and outcomes.

Comparison	Study ID	Time from surgery till recruitment	Site of surgery % / exclusions	Clinical relapse definition	Endoscopic / surgical/ radiologic/ histological relapse definition/
AZA & 6‐MP versus Placebo
AZA versus Placebo (12 months) both arms 750 mg/day metronidazole (3 months)	D'Haens 2008	2 weeks	Perforating disease 48 *Macroscopic evidence for CD proximally or distally to the site of resection or the presence of frank pancolitis or an ileorectal anastomosis, patients with a stoma; operation for fibrostenosis only	CDAI > 250	Rutgeerts i ≥ 2
6‐MP 50 mg/day versus Placebo (24 months)	Hanauer 2004	Before postoperative hospital discharge	N/A * Active perianal disease or any active disease in other segments of the intestine	Clinical recurrence grading > 2 (Hanauer)	Rutgeerts i ≥ 2 Radiographic relapse: Radiographic recurrence grading > 2
6‐MP 1 mg/kg/day versus Placebo (3 years)	Mowat 2016	≤ 3 months	Ileal 39; Colonic 2; Ileocolonic 59 * Need for further surgery, stricturoplasty alone, formation of a stoma	CDAI > 150 and a 100‐point increase from baseline	Rutgeerts i ≥ 2 HRQOL: IBDQ scores
AZA & 6‐MP vs 5‐ASA
AZA 2 mg/kg versus Mesalamine 3 mg/kg (24 months)	Ardizzone 2004	Maximum 2 weeks	Small bowel only 25.3; Colon 5.6; Small bowel and colon 9.8; upper gastrointestinal tract 16.2 *Surgical procedures other than conservative surgery or for perianal disease only	CDAI > 200 Surgical relapse: need for another surgical procedure	n/a
6‐MP 50 mg/day versus Mesalamine 3 g/day (24 months)	Hanauer 2004	Before postoperative hospital discharge	N/A * Active perianal disease or any active disease in other segments of the intestine	clinical recurrence grading > 2	Rutgeerts i ≥ 2 Radiographic relapse: radiographic recurrence grading > 2
AZA 2 mg/kg/day versus Mesalamine 4 g/day (12 months)	Reinisch 2010	6‐24 months	N/A * Short bowel syndrome, an ileocolonic stoma	CDAI > 200	Rutgeerts i ≥ 2 HRQOL: IBDQ
AZA 2 mg/kg/day versus Mesalazine 3 g/day (24 months)	Savarino 2013	2‐4 weeks	Ileum 49, Ileocolonic 51. * Fibrostenotic stricture, macroscopically active disease not resected at the time of surgery, and presence of a stoma	1. ≥ 2 on the clinical recurrence grading scale by Hanauer 2. CDAI > 200	Rutgeerts i≥2 Radiologic relapse: ≥ 2 radiographic recurrence grading scale HRQl: IBDQ>170
AZA & 6‐MP vs anti‐TNF‐α
AZA 2.5 mg versus Infliximab 5 mg/kg (12 months)	Armuzzi 2013	2‐4 weeks	Not reported *Active perianal disease, presence of stoma	HBI ≥ 8	Rutgeerts' score ≥ i2
AZA 2 mg/kg/day versus Adalimumab (24 months)	Savarino 2013	2‐4 weeks	Ileum 49, Ileocolonic 51. * Fibrostenotic stricture, macroscopically active disease not resected at the time of surgery, and presence of a stoma	1. ≥ 2 on the clinical recurrence grading scale by Hanauer 2. CDAI > 200	Rutgeerts i ≥ 2 Radiologic relapse: ≥ 2 radiographic recurrence grading scale HRQl: IBDQ > 170
AZA 2.5 mg/kg/day versus Adalimumab (52 weeks) both arms 750mg/day metronidazole (3 months)	Lopez‐Sanroman 2017	2 weeks	Ileal 58, ileocolonic 41 * Postsurgical stoma, resection for short indolent stenosis, inaccessible anastomosis to endoscopy	CDAI > 200	Rutgeerts i ≥ 2
6‐MP 1.5 mg/kg/day versus Adalimumab (12 months)	Scapa 2015	< 45 days	6‐MP 1.5 mg/kg/day vs Placebo (12 months)	Scapa 2015	< 45 days

AZA: azathioprine; 6‐MP: 6‐mecarptopurine; mg: milligram; CD: Crohn's disease; CDAI: Crohn's disease activity index; NA: not applicable; kg: kilogram; g: gram; HRQOL: health related quality of life; IBDQ: inflammatory bowel disease questionnaire; TNF: tumour necrosis factor; HBI: Harvey Bradshaw index; 5‐ASA: 5‐aminosalicylic acid