## 1 Introduction  
Endodontic therapy or root canal treatment (RCT) aims to eradicate bacteria from an infected root canal [1]. It is generally performed over multiple visits, which is considered a safer procedure than a single visit [2]. One or single visit RCT involves cleaning and shaping, and obturation of the root canal in one appointment; whereas multiple visit RCT provides a gap after placement of an intracanal medicament followed by obturation.  
Multivisit RCT has a few disadvantages. There is a risk of contamination of root canal due to microleakage and fracture of the temporary restoration that can cause a flare up [3; 4]. Patient needs and expectations have made one/single visit treatment more popular as it is brief and economical [5]. With recent advances in endodontic technology, single-visit endodontics is favoured [6; 7]. Factors like preoperative pain and pulpal infection that affect the treatment outcome, determine the appropriate treatment plan, e.g., single or multiple-visit [6]. The existing literature is unclear if either of the options offers an advantage over the other [2; 3; 5; 6].  
Pain after RCT is one of the most common complications of endodontic treatment [8]. Postoperative pain from RCT may occur between 3% to 50% of cases [8]. Mild postoperative pain is not rare even when endodontic treatment follows established protocols [9]. A flareup, here, refers to intense pain and/or swelling of the facial soft tissues and the oral mucosa in the area of endodontically treated tooth after initiation of endodontic treatment. The clinical symptoms (pain on biting, chewing or isolated) are so strongly expressed that the patient needs to visit the clinic sooner than scheduled [10; 11; 12]. After endodontic treatment, a flareup often manifests as pain of varying intensity with or without swelling [13]. A flareup can occur within a few hours or a few days post RCT [14]. The pain may be a periapical inflammatory response to one or more of the following factors: instrumentation/mechanical, the introduction of medications/chemical injury, apical extrusion of debris into the periapical tissues and psychological influences [15]. Factors such as preoperative symptoms, age, gender, type of tooth, dental anatomy, periapical lesions, sinus tract, tooth vitality, and intracanal medications are associated with the flareup [16; 17].  
Studies have compared both the incidence and the intensity of postendodontic pain and flareups between a single visit and multiple visit RCT. Reviews on this topic include varying study designs which causes significant heterogeneity in the included data making a quantitative analysis difficult. One review compared single and multiple visit RCT in the presence of apical periodontitis and included research articles of varying study designs. The second review compared the postoperative pain between single and multiple visit RCT but again included both randomised and nonrandomised controlled trials. The strongest scientific evidence is a synthesis of findings produced by well conducted, randomised controlled trials. Our research, therefore, assessed incidence and intensity of postendodontic pain and flareup between single visit versus multiple visit RCT from randomised controlled trials.