Transforaminal full-endoscopic lumbar discectomy (TELD) can be performed under local anesthesia. However, there have been no reports on risk factors for a change in vital signs or the need for additional medications to maintain adequate analgesia during this procedure. The purpose of this study was to identify risk factors for additional intravenous medication during TELD under local anesthesia. The following factors were retrospectively evaluated in 113 consecutive patients who underwent TELD under local anesthesia at our institution: demographic characteristics, radiological features at the intervertebral disc level, distance between the superior articular process and the exiting nerve root, height of the intervertebral disc, height of the bulging disc, height of the intervertebral foramen, and distance from the insertion site to the spinous process on magnetic resonance imaging (MRI) and computed tomography (CT) scans of the lumbar spine. Logistic regression analysis was performed to determine factors associated with the need for additional drugs. In all, 23 cases (20.4%) required additional intraoperative medications because of hypertension, hypotension, bradycardia, or pain. Logistic regression analysis revealed that age (partial regression coefficient 0.05, p = 0.02) and bulging disc height (partial regression coefficient -0.7, p = 0.003) influenced the need for additional drugs. There were significant associations of need for additional intravenous medication with older age (>62 years) and a smaller bulging disc height (<8.2 mm). Patients with these factors require close monitoring for changes in vital signs or increasing pain when performing TELD under local anesthesia and may need additional intravenous medication.