Nexium's side effects are mostly digestion-related in nature. Common side effects include: diarrhea or constipation, stomach pain or gas, nausea, dry mouth, or headaches. If these side effects are persistent or increasingly severe, notify your doctor. You should also contact your doctor immediately if you develop any of the following side effects: hoarse speech; irritation of the skin such as blisters or peeling, hives, rash, or itchiness; difficulty swallowing food or breathing; or swelling around the mouth, face, or limbs. These can be a signal of an allergic reaction to Nexium.