What is a common cause of swelling several days after root canal treatment?

Swelling several days after root canal treatment is often attributed to the presence of bacteria in the periapical region, which can occur due to entrapment during the procedure. Even if the root canal is adequately treated, residual bacteria or an infection that was not completely eliminated can proliferate, leading to inflammation and subsequent swelling. This is particularly relevant as roots may harbor bacteria in anatomical complexities or untreated areas, which can cause a localized infection post-treatment as the body responds to the bacterial presence.

The other options, while they may relate to complications in root canal therapy, do not typically result in delayed swelling. Overinstrumentation and underfilling could lead to immediate complications or discomfort but are less likely to cause swelling several days later compared to an entrenched infection. An allergic reaction to anesthetics usually presents immediately or soon after the anesthesia is administered, rather than manifesting days later in the form of swelling related to the root canal procedure.