Here, we present the case of a patient who initiated risperidone and developed persistent tachycardia, which was exacerbated by subsequent administration of paliperidone palmitate despite treatment with propranolol. This patient's experience emphasizes the need for psychiatrists to regularly monitor vital signs after risperidone/paliperidone initiation, dosage increase, or overdose to identify and appropriately manage this potentially harmful side effect. Increasing clinician awareness of this rare side effect will help protect patients with serious mental illness who regularly rely upon these medications, particularly in their long-acting injectable formulations.