# A 55-year-old client takes warfarin for atrial fibrillation, simvastatin for hyperlipidemia, and is now prescribed erythromycin 500 mg PO four times daily for a skin infection. Which client report should the nurse address as a priority drug interaction concern? > source: MyMerci (mymerci.kr) > url: https://mymerci.kr/pages/nclex_q.php?qn_id=375233&lang=en > language: en > subject: Medical Emergencies > category: PA ## Question A 55-year-old client takes warfarin for atrial fibrillation, simvastatin for hyperlipidemia, and is now prescribed erythromycin 500 mg PO four times daily for a skin infection. Which client report should the nurse address as a priority drug interaction concern? ## Option 1. A mild bilateral headache that started shortly after beginning the antibiotic, responsive to rest and not accompanied by blurred vision, dizziness, or any focal weakness; the client reports it as a "tight band" sensation and denies any recent trauma or change in warfarin dose. The headache does not interfere with daily activities and has not increased in intensity over three days. 2. Bruising, increased bleeding (warfarin INR may rise via CYP3A4 inhibition) and new muscle pain or weakness (simvastatin level may rise leading to rhabdomyolysis) plus the well-known severe GI cramping and nausea from erythromycin acting as a motilin receptor agonist; communicate with the prescriber, anticipate INR check and statin hold or change to a non-CYP3A4 antibiotic. **✔ Correct answer** 3. A persistent metallic taste in the mouth noticed immediately after taking erythromycin, described as "like sucking on a penny," which lingers for about an hour and is not accompanied by any nausea, vomiting, or difficulty swallowing; oral intake remains adequate and the client has no known allergies; the taste has persisted for two days without worsening, and no tongue swelling, rash, or breathing difficulty. 4. A flat, non-itchy, pink rash on the upper chest that appeared on day three of antibiotic therapy, which blanches with pressure and is not accompanied by fever, shortness of breath, or swelling of the lips or tongue; the client recently changed laundry detergent and had a similar rash in the past that resolved without treatment. **Correct answer: 2** ## Explanation Erythromycin is a macrolide that is a strong CYP3A4 inhibitor (clarithromycin is also strong; azithromycin is weak). It is also a motilin receptor agonist, producing significant nausea, vomiting, abdominal cramping, and diarrhea — sometimes used at low dose as a prokinetic. The clinical priorities in this scenario: (1) warfarin — erythromycin inhibits CYP3A4 and 1A2 metabolism, raising INR and bleeding risk; bruising and bleeding are warning signs; anticipate INR monitoring; (2) simvastatin — strong CYP3A4 substrate; erythromycin elevates simvastatin levels markedly, causing myopathy and rhabdomyolysis; new muscle pain, weakness, dark urine require holding the statin and CK measurement; (3) GI motilin effect — severe cramping, nausea, vomiting, diarrhea common; can be limiting; report and titrate; (4) QT prolongation as macrolide class effect; (5) hepatotoxicity rare but possible; (6) several other interactions — carbamazepine, theophylline, cyclosporine, and many others — require pharmacist review. Nursing actions: communicate with prescriber and pharmacy, anticipate INR check, possible statin hold or switch to a non-CYP3A4-inhibiting antibiotic, education on bleeding and myopathy signs, supportive care for GI side effects. ## In-depth explanation Erythromycin = strong CYP3A4 inhibitor + motilin agonist. Warfarin and statin interactions are highest priority. 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