A 32-year-old client on chronic fluoxetine for major depression presents with new agitation, sweating, tremor, hyperreflexia with clonus, T 39.4°C, HR 130, BP 168/96, after starting sumatriptan and tramadol for migraine plus an OTC dextromethorphan cold remedy. Which is the priority nursing action?

Serotonin syndrome arises from excess serotonergic activity, often from drug combinations. Diagnostic features (Hunter criteria): mental status changes (agitation, confusion), neuromuscular hyperactivity (hyperreflexia, clonus including ocular clonus, tremor, rigidity), and autonomic instability (hyperthermia, tachycardia, hypertension, diaphoresis, mydriasis, GI symptoms). Onset typically within hours of the inciting combination. Common offenders: SSRI/SNRI/TCA, MAOIs (including linezolid, methylene blue, recent rasagiline/selegiline), triptans, tramadol, meperidine, dextromethorphan, ondansetron, St John wort, MDMA, fentanyl, lithium. Standard nursing response: (1) recognize and immediately stop all serotonergic agents; (2) support — IV fluids, cooling for hyperthermia (active cooling for >38.5C; intubation/paralysis if severe), benzodiazepines for agitation and to reduce muscle activity; (3) cyproheptadine 12 mg PO/NG initial then 2 mg every 2 hours, max 32 mg/day, for moderate-to-severe symptoms; (4) avoid antipyretics like acetaminophen alone (centrally driven hyperthermia) and physical restraints (worsen muscle activity); (5) ICU transfer if severe; (6) review home OTC and herbals; educate about SSRI washout when planning future serotonergic therapy. Continuing, adding SSRI, naloxone alone are unsafe.

Hyperreflexia with clonus + autonomic instability + serotonergic stack = serotonin syndrome. Stop everything serotonergic, support, benzodiazepines, cyproheptadine for moderate-severe.