Renal Safety of Topical Diclofenac in Adults with Increased Renal Risk: A Systematic Review.

Background and Objectives: Topical diclofenac is recommended over oral non-steroidal anti-inflammatory drugs (NSAIDs) for osteoarthritis, particularly in older adults and those with comorbidities, on the assumption that low systemic absorption limits renal effects. Whether this presumed safety extends to patients already at increased risk of kidney injury has not been systematically evaluated. This review assessed the evidence on renal outcomes of topical diclofenac in adults. Materials and Methods: We conducted a systematic review (PROSPERO CRD420261393454) of studies reporting renal outcomes after topical diclofenac exposure. PubMed, Embase, Web of Science, and Scopus were searched from inception to 19 April 2026. Studies were stratified a priori into increased-renal-risk and general populations and synthesised separately, without pooling. Risk of bias was assessed with RoB 2, ROBINS-I, and JBI tools, certainty with GRADE, and synthesis followed the SWiM framework. Results: Eighteen studies, contributing data from more than 500,000 participants, were included; 14 underwent primary risk-of-bias appraisal (three at low, three at serious or high risk). In general populations, topical diclofenac produced little to no change in serum creatinine or creatinine clearance and smaller renal effects than oral diclofenac, providing high-certainty evidence of a favourable profile. In increased-renal-risk populations, one adjusted cohort reported higher acute kidney injury (AKI) risk among topical NSAID users, although exposure was predominantly to non-diclofenac agents; within the same cohort, topical NSAIDs carried lower risk than systemic NSAIDs. No study evaluated early renal injury biomarkers; certainty was moderate owing to indirectness. Conclusions: Relative to oral diclofenac, topical diclofenac shows a favourable renal safety profile, with high-certainty evidence in general populations. In adults at increased renal risk, moderate-certainty evidence derived from predominantly non-diclofenac exposure over short observation windows cannot exclude a modest excess of any-stage AKI; renal risk therefore appears reduced rather than absent. Diclofenac-specific studies in chronic kidney disease using sensitive biomarkers and longer follow-up are needed.