Delayed reactions after hyaluronic acid injections: a review for physicians outside aesthetic medicine

Background. Delayed reactions after hyaluronic acid (HA) injections are rare but clinically significant complications that may occur months or years after aesthetic procedures. Their nonspecific presentation may mimic infections, cysts, salivary gland disorders, or neoplasms, causing diagnostic difficulties for physicians outside aesthetic medicine. Aim. To review current evidence on the epidemiology, pathogenesis, clinical presentation, diagnosis, and management of delayed reactions after HA injections, with emphasis on practical aspects for non-aesthetic physicians. Material and methods. A narrative review was conducted using PubMed publications from 2015–2026 and the ISAPS report. Reviews, expert consensus statements, case reports/series, observational studies, post-marketing surveillance analyses, and diagnostic/therapeutic algorithms were analyzed. Results. Delayed reactions after HA fillers include swelling, erythema, induration, inflammatory and non-inflammatory nodules, granulomas, and delayed infections. Pathogenesis is multifactorial, involving foreign body reactions, immune-mediated mechanisms, and infectious processes, including biofilm formation. Manifestations may be triggered by infections, vaccination, trauma, or dental procedures. Diagnosis relies on history, clinical examination, and differential diagnosis, with imaging, microbiological testing, or histopathology needed in selected cases. Treatment depends on the mechanism and may include antibiotics, hyaluronidase, corticosteroids, or procedural interventions. Conclusions. Delayed reactions after HA injections should be considered in the differential diagnosis of persistent facial nodules, swelling, and inflammatory lesions, even years after cosmetic procedures. Routine inquiry about previous aesthetic treatments is essential to avoid misdiagnosis.