PurposeRadical cystectomy (RC) is the standard treatment for high-risk non muscle-invasive bladder cancer (NMIBC) failing first BCG treatment. A second BCG course is an option for those patients who refuse RC or are not eligible for it, but its success rate is quite low. Aim of the present study was to determine whether the addition of intravesical electromotive drug administration of mytomicin-C (EMDA-MMC) improved the efficacy of second BCG course.MethodsPatients with high-risk NMIBC having failed first BCG treatment and having refused RC were offered a second BCG induction course either alone (group A) or combined with EMDA-MMC (group B). Recurrence-free survival (RFS), progression-free survival (PFS) and cancer-specific survival (CSS) were tested.ResultsOf the 80 evaluable patients, 44 were in group A and 36 in group B; median follow-up was 38 months. RFS was significantly worse in group A whereas there was no difference in PFS and CSS between the two groups. Stratifying by disease stage, Ta patients receiving combined treatment had statistically better RFS and PFS survival than those receiving BCG only; this difference did not apply to T1 patients. Multivariable analysis confirmed that combined treatment was a significant predictor of recurrence and was close to predict progression. No tested variable was predictive of recurrence or progression in T1 tumours. Among those who underwent RC, CSS was 61.5% in those who had progression and 100% in those who remained with NMIBC.ConclusionCombined treatment improved RFS and PFS only in patients with Ta disease.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00345-023-04372-5.