AbstractAimThe aim of the study was to assess the tolerance to the new root canal irrigation fluid RISA after root canal treatment (RCT) by evaluating the subject's postoperative well‐being, postoperative pain (PP) and treatment outcome.MethodologyA single‐arm prospective study with 16 subjects (17 teeth) diagnosed with asymptomatic apical periodontitis. Endodontic treatment in one session performed using RISA for root canal irrigation. Well‐being was assessed on the same day and after 24 h by telephone. For pain intensity, a visual analogue scale was used at 0–5 days. Clinical and radiographic evaluations were performed at ≥12 months. Well‐being, occurrence of PP and outcome were qualitatively reported. Friedman test for paired samples and Spearman correlation coefficient were used. Significance was set at p < .05.ResultsAt the same day and after 24 h, 14/16 subjects felt ‘good’. 9/16 presented intra‐ or extra‐oral swelling. The frequency of PP ≥36 (weak) was 82.4%. On the same day, 1 and 2 days postoperatively, there was more pain compared with preoperative pain p < .05. At Day 3, PP equalled preoperative pain (p > .05). 62.5% of subjects needed analgesics Day 0–2. The recall rate was 94.1%, and resolution of apical periodontitis was observed in 87.5%.ConclusionsThe well‐being of subjects was good, and the overall PP intensity was low. However, postoperative intra‐ and extra‐oral swelling occurred often. At the recall visit, the effectiveness of the RCT with RISA appeared high (87.5%). The encouraging outcome results plus the fact that RISA has a broader action range than NaOCl in vitro, justify further work on the RISA solution. To reduce postoperative swelling, it is advised to further investigate the optimal way of application of RISA in the laboratory before clinical application is recommended.