Blood glucose tests and cholesterol tests require patients to avoid eating or drinking anything except water for 8 hours before the test, unless directed otherwise by a doctor, as stated by Geisinger Medical Laboratories. Patients can usually eat normally before other types of blood tests.
Patients who plan to get blood drawn should drink plenty of water to keep their blood pressure up. If eating is permitted before the test, eating a meal a few hours prior to the test can prevent lightheadedness or dizziness, according to Geisinger Medical Laboratories. Caffeinated beverages such as coffee are not recommended, as they can cause the body to lose water.