advocates the use of an antibiotic mixture instead of root canal instrumentation. Case Report: A 9-year-old female patient sought consultation for spontaneous pain associated with tooth 36, with a sudden and pulsatile onset, which decreased with analgesics and increased at night, during chewing, with thermal and mechanical stimuli, hindering detailed tooth brushing. Clinically, the tooth presented marked opacities associated with an active and cavitated dental caries lesion and post-eruptive fracture. Radiographically, a radiolucent image was observed in the furcation region with open apices and radiolucent areas in the mesial and distal roots. Tooth 36 was diagnosed with severe Molar-Incisor Hypomineralization (MIH) + symptomatic irreversible pulpitis and symptomatic apical periodontitis. LSTR therapy was performed using a triantibiotic paste based on chloramphenicol, tetracycline, and zinc oxide and eugenol (CTZ Paste). The tooth was restored with a preformed stainless-steel crown using the modified Hall technique. Over the 24-month follow-up, the patient reported no painful symptoms, and no signs of inflammation or infection were observed. Radiographically, there was an increase in root length, apical closure, and bone repair in the furcation and periapical regions. Conclusion: LSTR therapy with CTZ Paste was effective in the treatment of a first permanent molar with irreversible pulpitis.