Clarithromycin, a widely used macrolide antibiotic, is generally well tolerated but can rarely cause thrombophlebitis following intravenous administration. We present a case of a 24-year-old male diagnosed with pneumonia, treated with intravenous clarithromycin and piperacillin-tazobactam. While his respiratory symptoms improved, he developed sudden-onset pain, swelling, and tenderness in his left upper limb on the third day of treatment. Doppler ultrasound confirmed cephalic vein thrombophlebitis, with patent deep veins. The patient was successfully managed with subcutaneous fondaparinux and oral antibiotics, leading to full recovery. This case highlights the need for vigilance in detecting clarithromycin-induced thrombophlebitis, as early intervention can prevent complications.