Final answer:
When starting sumatriptan, a medication for migraines, the patient should be instructed to monitor for chest pain, a side effect necessitating medical attention due to its association with serious cardiac events.
Step-by-step explanation:
The nurse should instruct the patient to monitor for and report chest pain to the doctor when starting to take sumatriptan. Sumatriptan, used for treating migraines, can have various side effects such as nausea, vomiting, fatigue, dizziness, and, in some cases, more serious cardiac effects. These cardiac effects can include increased heart rate, hypertension, and an increased probability of heart attacks, which is why chest pain is particularly important to monitor and report.