A client who experiences migraine headaches reports having fewer headaches since using the herbal remedy feverfew. Which information is most important for the nurse to include in a teaching plan for this client?

A. While feverfew can cause gastrointestinal issues, increased anxiety and nervousness are not commonly reported side effects. B. Feverfew may cause allergic reactions in individuals with sensitivities to related plants like chamomile, ragweed, or yarrow, making this information crucial for preventing allergic reactions. C. Gastrointestinal side effects like abdominal pain, gas, nausea, vomiting, and diarrhea are possible but are not as critical as the risk of an allergic reaction. D. Interaction with aspirin or NSAIDs is a concern but is secondary to the importance of avoiding feverfew in individuals with known allergies to similar plants.