Trismus may be a complication from local anesthesia. Patients with trismus of unknown cause after dental treatment should be evaluated thoroughly. The dentist should perform a complete examination and establish a differential diagnosis to avoid missing a serious or life-threatening infection. A right lateral pharyngeal space infection developed after a general restorative procedure that involved an inferior alveolar nerve injection on the same side. The patient's severe trismus required awake intubation, incision and drainage while the patient was under general anesthesia, treatment with antibiotics and ten days of hospitalization.