Lichen Planus and Lichenoid Reactions Associated with IL-17 Inhibition: A Comprehensive Review of Clinical Phenotypes, Latency, Histopathology, Immunopathogenesis, and Clinical Outcomes

Background: Lichen planus (LP) is a T-cell-mediated inflammatory disorder involving the skin, mucosae, nails and hair follicles. Published observations of LP and lichenoid reactions during interleukin-17 (IL-17) inhibition pose diagnostic and causality challenges, particularly because IL-17 blockade has also been associated with clinical improvement in some forms of LP. Aim: To review the clinical phenotypes, timing, histopathological features, immunopathogenic context and outcomes of LP-like reactions reported during IL-17 inhibitor treatment. Methods: This narrative review examines the clinical literature discussed in the manuscript, with particular attention to secukinumab, ixekizumab, brodalumab and bimekizumab, and considers alternative explanations for suspected treatment-associated disease. Results: Reported manifestations include cutaneous eruptions and oral or other mucosal disease. Onset has been described from less than one week to several months following treatment initiation. Histopathology can support a lichenoid diagnosis but cannot independently establish drug causality. Relevant competing factors include pre-existing LP, concomitant medicines, hepatitis C infection and local exposures. Treatment withdrawal, continuation and symptomatic therapy have been followed by varied clinical courses. Available observations are predominantly case-based and do not provide reliable estimates of incidence or comparative risk between inhibitors. Conclusion: Careful clinicopathological evaluation, precise chronological documentation and individualised assessment of alternative causes are essential. More consistently reported cases and studies with exposure denominators are required to clarify causality and comparative safety.