An Unusual Iatrogenic Complication of Botulinum Toxin: Forearm Flexor Weakness Following a Nonstandard Intradermal Sensitivity Test

Botulinum toxin (BoNT) type A is widely used in aesthetic and functional medicine and is generally considered safe when administered at appropriate doses and using proper techniques. However, rare and unusual complications may occur, even in patients with no previous neurological or allergic history. The author reports the case of a 24-year-old female who developed an unexpected neuromuscular complication characterized by progressive weakness of the forearm flexor muscles following a nonstandard intradermal sensitivity test of BoNT performed on the volar aspect of the forearm. The procedure was carried out by a nonspecialist practitioner. The patient subsequently developed reduced grip strength and functional limitation of hand movements within days, without systemic symptoms or clinical evidence of an allergic reaction. Clinical evaluation suggested a pharmacological neuromuscular effect rather than an immunologically mediated hypersensitivity reaction. No cutaneous signs of allergy, such as urticaria or angioedema, were observed. The temporal relationship and anatomical distribution of weakness were consistent with possible local toxin diffusion or unintended neuromuscular junction blockade. The patient was managed conservatively; a gradual improvement in muscle strength was observed over time. This case highlights a rare and atypical complication associated with the nonstandard use of BoNT and emphasizes the importance of adherence to evidence-based injection protocols performed by qualified specialists. It also raises concerns regarding the practice of intradermal "sensitivity testing," which lacks scientific validation and may lead to unintended neuromuscular effects, particularly when performed by nonspecialists. Further studies are needed to clarify the safety implications of nonstandard applications and to distinguish true hypersensitivity reactions from pharmacologic diffusion-related effects. Level of Evidence: 5 (Therapeutic) For image description, please refer to the figure legend and surrounding text.