Ref (type)	Population	Outcome, Interventions	Results and statistical analysis	Effect size	Favours
Retention in treatment
[20] Systematic review	505 people 3 RCTs in this analysis	Proportion of people retained in treatment 173/254 (68%) with methadone 63/251 (25%) with control	RR 3.05 95% CI 1.75 to 5.35 Significant heterogeneity was found between RCTs (P = 0.02)	Moderate effect size	methadone
[20] Systematic review	750 people 4 RCTs in this analysis	Proportion of people retained in treatment 318/433 (73%) with methadone 52/317 (16%) with control	RR 4.44 95% CI 3.26 to 6.04 P <0.00001	Moderate effect size	methadone