Yes. All herbals have the potential of altering the actions of prescription drugs. Always ask your pharmacist to run a cross check when ingesting anything; including foods.
Yes. Echinacea might interact with corticosteroids, antifungals, anesthetics, and some antibiotics. It might increase toxicity to the liver if you're taking something potentially toxic to that organ. Keep in mind that echinacea is often used to boost the immune system, and medicines like corticosteroids or cancer medicines (prednisone etc.) are meant to suppress it. Always check with your doctor.