Inclusion Criteria:  age 12 to 18 years; presenting chief complaint of headache, head injury, or concussion within the first 48 hours of injury; GCS > 13 on arrival  Exclusion Criteria:  age < 12 years or > 18 years; inability to provide informed consent; vomiting > 2 episodes following injury; physical or mental disability hindering adequate response to assessment of symptoms; hemodynamic instability/medical condition requiring further acute life-saving medical intervention; known brain mass, intracranial hemorrhage, skull fracture; known contraindications to magnesium use; known contraindications to ondansetron use; pregnant patients