BACKGROUND: It remains unclear whether people with non-muscle invasive bladder cancer (NMIBC) benefit from intravesical gemcitabine compared to other agents in the primary or recurrent setting following transurethral resection of a bladder tumor. This is an update of a Cochrane Review first published in 2012. Since that time, several randomized controlled trials (RCTs) have been reported, making this update relevant. OBJECTIVES: To assess the comparative effectiveness and toxicity of intravesical gemcitabine instillation for NMIBC. SEARCH METHODS: We performed a comprehensive literature search of the Cochrane Library, MEDLINE, Embase, four other databases, trial registries, and conference proceedings to 11 September 2020, with no restrictions on the language or status of publication. SELECTION CRITERIA: We included RCTs in which participants received intravesical gemcitabine for primary or recurrent NMIBC. DATA COLLECTION AND ANALYSIS: Two review authors independently assessed the included studies and extracted data for the primary outcomes: time to recurrence, time to progression, grade III to V adverse events determined by the Common Terminology Criteria for Adverse Events version 5.0 (CTCAE v5.0), and the secondary outcomes: time to death from bladder cancer, time to death from any cause, grade I or II adverse events determined by the CTCAE v5.0 and disease-specific quality of life. We performed statistical analyses using a random-effects model and rated the certainty of the evidence using GRADE. MAIN RESULTS: We included seven studies with 1222 participants with NMIBC across five comparisons. This abstract focuses on the primary outcomes of the three most clinically relevant comparisons. 1. Gemcitabine versus saline: based on two years' to four years' follow-up, gemcitabine may reduce the risk of recurrence over time compared to saline (39% versus 47% recurrence rate, hazard ratio [HR] 0.77, 95% confidence interval [CI] 0.54 to 1.09; studies = 2, participants = 734; I(2) = 49%; low-certainty evidence), but the CI included the possibility of no effect. Gemcitabine may result in little to no difference in the risk of progression over time compared to saline (4.6% versus 4.8% progression rate, HR 0.96, 95% CI 0.19 to 4.71; studies = 2, participants = 654; I(2) = 53%; low-certainty evidence). Gemcitabine may result in little to no difference in the CTCAE grade III to V adverse events compared to saline (5.9% versus 4.7% adverse events rate, risk ratio [RR] 1.26, 95% CI 0.58 to 2.75; studies = 2, participants = 668; I(2) = 24%; low-certainty evidence). 2. Gemcitabine versus mitomycin: based on three years' follow-up (studies = 1, participants = 109), gemcitabine may reduce the risk of recurrence over time compared to mitomycin (17% versus 40% recurrence rate, HR 0.36, 95% CI 0.19 to 0.69; low-certainty evidence). Gemcitabine may reduce the risk of progression over time compared to mitomycin (11% versus 18% progression rate, HR 0.57, 95% CI 0.32 to 1.01; low-certainty evidence), but the CI included the possibility of no effect. We are very uncertain about the effect of gemcitabine on the CTCAE grade III to V adverse events compared to mitomycin (RR 0.51, 95% CI 0.13 to 1.93; very low-certainty evidence). The analysis was only based on recurrent NMIBC. 3. Gemcitabine versus Bacillus Calmette-Guérin (BCG) for recurrent (one-course BCG failure) high-risk NMIBC: based on 6 months' to 22 months' follow-up (studies = 1, participants = 80), gemcitabine may reduce the risk of recurrence compared to BCG (41% versus 97% recurrence rate, HR 0.15, 95% CI 0.09 to 0.26; low-certainty evidence) and progression over time (16% versus 33% progression rate, HR 0.45, 95% CI 0.27 to 0.76; low-certainty evidence). We are very uncertain about the effect of gemcitabine on the CTCAE grade III to V adverse events compared to BCG (RR 1.00, 95% CI 0.21 to 4.66; very low-certainty evidence). In addition, the review provides information on the comparison of gemcitabine versus BCG and gemcitabine versus one-third dose BCG. AUTHORS' CONCLUSIONS: Based on findings of this review, gemcitabine may have a more favorable impact on recurrence and progression-free survival than mitomycin but we are very uncertain as to how major adverse events compare. The same is true when comparing gemcitabine to BCG in individuals with high risk disease who have previously failed BCG. The underlying low- to very low-certainty evidence indicates that our confidence in these results is limited; the true effects may be substantially different from these findings; therefore, better quality studies are needed.