Risks from venipuncture include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Interfering factors:

• Acute stress (for example, from surgery or an infection)
• Vigorous exercise

Several drugs may cause glucose intolerance, including the following:

• Thiazide diuretics (e.g., hydrochlorothiazide)
• Beta-blockers (e.g., propranolol)
• Oral contraceptives
• Corticosteroids (e.g., prednisone)
• Some psychiatric medications

Before having the test, let your health care provider know if you are taking any of these medications.