An empty stomach is essential for a safe and certain examination, so you should have nothing to eat or drink, including water, for approximately six hours before the examination. Your doctor will present more specific details about this, depending on the time of day that your test is scheduled. It is better to tell your doctor in advance about any medications you take. You can need to adjust your usual dose for the examination. This is particularly crucial if you have diabetes. Please discuss any allergies to medications and medical conditions such as heart or lung disease.