Abstract: We included eight studies involving 4488 participants. Regarding quality of evidence, trials showed differences in study population, study design, type of antibiotic, and definition of infection; however, primary outcomes among the included studies were consistent. Mortality rate in the preventive antibiotic group was not significantly different from that in the control group (373/2208 (17%) vs 360/2214 (16%); RR 1.03, 95% confidence interval (CI) 0.87 to 1.21; high-quality evidence). The number of participants with a poor functional outcome (death or dependency) in the preventive antibiotic therapy group was also not significantly different from that in the control group (1158/2168 (53%) vs 1182/2164 (55%); RR 0.99, 95% CI 0.89 to 1.10; moderate-quality evidence). However, preventive antibiotic therapy did significantly reduce the incidence of 'overall' infections in participants with acute stroke from 26% to 19% (408/2161 (19%) vs 558/2156 (26%); RR 0.71, 95% CI 0.58 to 0.88; high-quality evidence). This finding was highly significant for urinary tract infections (81/2131 (4%) vs 204/2126 (10%); RR 0.40, 95% CI 0.32 to 0.51; high-quality evidence), whereas no preventive effect for pneumonia was found (222/2131 (10%) vs 235/2126 (11%); RR 0.95, 95% CI 0.80 to 1.13; high-quality evidence). No major side effects of preventive antibiotic therapy were reported. Only two studies qualitatively assessed the occurrence of elevated body temperature; therefore, these results could not be pooled. Only one study reported length of hospital stay. Preventive antibiotics had no effect on functional outcome or mortality, but significantly reduced the risk of 'overall' infections. This reduction was driven mainly by prevention of urinary tract infection; no effect for pneumonia was found.

Plain-language summary: We included eight studies on preventive antibiotic therapy, with a total of 4488 people with stroke: 2230 participants were randomised to preventive antibiotic therapy, and 2258 to control. The mean age of participants in the preventive antibiotics group was 74.2 years, and in the control group 74.8 years. In both groups, the percentage of men was 52%. Study interventions differed in all eight studies; in two studies, trialists selected the (type of) antibiotic according to local antibiotic policy, with the aim of treating pneumonia. Preventive antibiotic treatment did not reduce the risk of dependency or death. However, preventive antibiotic therapy did significantly reduce the occurrence of 'overall' infections from 26% to 19%. Regarding type of infection, findings were highly significant for urinary tract infections (4% vs 10%) but showed no effect on pneumonia (10% vs 11%). No major side effects of preventive antibiotic therapy were reported. It has become possible to draw first 'overall' conclusions on the net effect of preventive antibiotic therapy in stroke; however, the decision of whether to use preventive antibiotic therapy in acute stroke should be reached with care. Studies were heterogeneous, and despite the large numbers of participants, results from a total of eight studies are limited. In two of these studies, risk of bias was considered to be high for three out of six criteria. Overall, reviewers considered the quality of evidence for the main outcomes of this review - looking at 'any' preventive antibiotic therapy, in 'any' dose, at any length of treatment - as high to moderate.