Objective:To test whether oral or va ginal lactobacillus can prevent vulvovaginitis after antib iotic treatment.Design:Randomised,placebo controlled,double blind,factorial2x2trial.Setting:Fifty general pr actices and 16pharma-cies in Melbourne,Australia.Parti cipants:Non -pregnant women aged 18-50years who required a short course of oral antibiotics for a non -gynaecological infection:278were enrolled in the study,and resul ts were available for235.Interventions:Lactobacillus preparations taken orally or vaginally,or both,from enrolmen t until four days after completion of their antibiotic cour se.Main outcome mea-sures:Participants’reports of symptoms of post-antibio tic vulvovaginitis,with microbiological evidence of candidiasis provided by a self obtained vaginal s wab.Results:Over-all,55/235(23%(95%confidence interval 18%to29%))women developed post -antibiotic vu lvovaginitis.Compared with placebo,the odds ratio for developing post -antibiotic vulvovaginitis wi th oral lactobacillus was1.06(95%confidence interval 0.58to 1.94)and with vaginal lactobacillus 1.38(0.75to 2.54).Compliance with antibiotics and interventions was high.The trial was terminated after the second interim analysis because of lack of effect of the interventions.Given the data at this time,the chances of detecting a significant reduction in vulvo-vaginitis with oral or vaginal lacto bacillus treatment were less than 0.032and 0.0006respectiv ely if the trial pro-ceeded to full enrolment.Conclusio ns:The use of oral or vaginal forms of lactobacillus to pr event post -antibiotic vulvovaginitis is not supported by t hese results.Further research on this subject is unlikely to be fruitful,unless new understandings about the pathogenesis of post -an-tibiotic vulvovaginitis indicate a possible role for lacto-bacillus. Objective: To test whether oral or va ginal lactobacillus can prevent vulvovaginitis after antibiotic treatment. Design: Randomized, placebo controlled, double blind, factorial 2 × 2 trial. Fifty general pr actices and 16 pharma-cies in Melbourne, Australia. Parti cipants: Non- pregnant women aged 18-50years who required a short course of oral antibiotics for a non-gynecological infection: 278were enrolled in the study, and resul ts were available for 235. Interventions: Lactobacillus preparations taken orally or vaginally, or both, from enrolmen t until four days after completion of their antibiotic cour se.Main outcome mea-sures: Participants’ reports of symptoms of post-antibio tic vulvovaginitis, with microbiological evidence of candidiasis provided by a self obtained vaginal s wab. Results: Over-all, 55 / 235 (23% (95% confidence interval 18% to 29%)) women developed post -antibiotic vu lvovaginitis. Compared with placebo, the odds ratio for developing post -antibiotic vulvovaginitis wi th oral lactobacillu s was 1.06 (95% confidence interval 0.58 to 1.94) and with vaginal lactobacillus 1.38 (0.75 to 2.54). Compliance with antibiotics and interventions was high. The trial was terminated after the second interim analysis because of lack of effect of the interventions. Given the data at this time, the chances of detecting a significant reduction in vulvo-vaginitis with oral or vaginal lacto bacillus treatment were less than 0.032 and 0.0006 elaborated ely if the trial pro-ceeded to full enrolment. Confluence ns: The use of oral or vaginal forms of lactobacillus to pr event post -antibiotic vulvovaginitis is not supported by t hese results. Focus on research on this subject is unlikely to be fruitful, unless new understandings about the pathogenesis of post-an-tibiotic vulvovaginitis indicate a possible role for lacto -bacillus.