2. Reasons for dropouts.

Study	N	time point	intervention	Reasons for drop out
				Intervention	Control (continue on AD monotherapy)
Appelberg 2001	108	6 weeks	Busiprone (augmentation of AD)	Protocol violation n = 3 Side effects n = 1 Moved house n = 1 Did not attend final assessment n = 1	Protocol violation n = 3 Did not attend final assessment n = 2 Stopped taking study medication n = 2 Spouse threw away study medication n = 1 Suspected angina n = 1 Heavy alcohol misuse n = 1
Dunner 2007	64	8 weeks	Ziprasidone (augmentation of AD)	80 mg/d: Treatment‐related adverse event n = 9 Withdrew consent n = 1 Miscellaneous n = 1 160 mg/d: Treatment‐related adverse event n = 7 Withdrew consent n = 1 Miscellaneous n = 2	Withdrew consent n = 4 Miscellaneous n = 1
Durgam 2016	819	8 weeks	Cariprazine (augmentation of AD)	1 mg/d to 2 mg/d: Adverse event n = 18 Insufficient response n = 4 Protocol violation n = 10 Withdrew consent n = 13 Lost to follow‐up n = 2 2 mg/d to 4.5 mg/d: Adverse event n = 36 Protocol violation n = 9 Withdrew consent n = 14 Lost to follow‐up n = 4	Adverse event n = 8 Insufficient response n = 2 Protocol violation n = 6 Withdrew consent n = 11 Lost to follow‐up n = 2 Other n = 2
El‐Khalili 2010 (PEARL)	446	6 weeks	Quetiapine (augmentation of AD)	150 mg/d: Adverse event n = 16 Lack of therapeutic response n = 2 Severe non‐compliance with study protocol n = 2 Did not complete ≥ 36 days of study treatment n = 1 Lost to follow‐up n = 8 Not willing to continue n = 4 Eligibility criteria not fulfilled n = 1 300 mg/d: Adverse event n = 27 Did not complete ≥ 36 days of study treatment n = 1 Lost to follow‐up n = 7 Not willing to continue n = 6 Eligibility criteria not fulfilled n = 1 Other n = 3	Adverse event n = 1 Lack of therapeutic response n = 4 Lost to follow‐up n = 10 Not willing to continue n = 8
Ferreri 2001	104	6 weeks	Mianserine (switched from AD) Mianersine (augmentation of AD)	Switched to mianserin alone: Ineffectiveness n = 1 Lost to follow‐up n = 2 Intolerance/ adverse events n = 8 Substantial improvement n = 0 Personal reason n = 1 Augmented with mianserin: Ineffectiveness n = 1 Lost to followup n = 2 Intolerance/ adverse events n = 2 Substantial improvement n = 1 Personal reason n = 1	Ineffectiveness n = 6 Lost to follow‐up n = 1 Intolerance/ adverse events n = 0 Substantial improvement n = 0 Personal reason n = 1
Kessler 2018	480	52 weeks	Mirtazapine (augmentation of AD)	Withdrew from study n = 6 Not willing to take part n = 2 Health n = 1 Other n = 3 Lost to follow‐up n = 44	Withdrew from study n = 12 Not willing to take part n = 8 Health n = 2 Other n = 2 Lost to follow‐up n = 28
McIntyre 2007	58	6 weeks	Quetiapine (augmentation of AD)	Adverse events n = 8 Consent withdrawal n = 2 Protocol violation n = 1	Lack of efficacy n = 9 Adverse events n = 2 Lost to follow‐up n = 2
Papakostas 2015	139	8 weeks	Ziprasidone (augmentation of AD)	Intolerance n = 10 Inefficacy n = 3 Lost to follow‐up n = 3 Other n = 6	Inefficacy n = 1 Lost to follow‐up n = 7 Other n = 7
Shelton 2001	20	8 weeks	Olanzapine (augmentation of AD)	Protocol violation n = 1	Lack of efficacy n = 1 Personal conflict n = 1 Protocol violation n = 1

^{AD: antidepressant}