Thirteen RCTs (n=2,996) were included in the review (the number of participants ranged from 111 to 443).
Eleven of the 13 studies scored 3 points out of a possible 5 for quality; given this low variation, the impact of methodological quality on treatment effect was not investigated.
Modifiers of the post-treatment EF domain score in the placebo group: patients who had a higher baseline EF domain score (erectile dysfunction of minor severity) showed a greater placebo response (p<0.001). No other confounders such as treatment duration, age of patients, aetiology of ED or ED duration had a statistically significant effect.
Modifiers of the post-treatment EF domain score in the sildenafil group: no confounders were shown to have a statistically significant effect.
Modifiers of treatment effect: trials where EF was less severe in patients at baseline showed a significantly lower effect (p<0.001); trials in patients with a longer history of ED showed a significantly greater effect (p=0.0018). Further analysis showed that treatment effect was strongly associated with the post-treatment EF domain score of the placebo group (p<0.001) but not with the post-treatment EF domain score of the sildenafil group.
Distorting effect modifiers: older trials showed a greater effectiveness of the sildenafil treatment than trials published more recently (p=0.006).
Publication bias: the funnel plot and Egger's test suggested there was no evidence of publication bias (p=0.858).