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.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Factors that may affect the test results:

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

Several drugs may cause glucose intolerance, including:

• Beta-blockers (for example, propranolol)
• Certain psychiatric medications
• Corticosteroids (for example, prednisone)
• Oral contraceptives (birth control pills)
• Thiazide diuretics (for example, hydrochlorothiazide)

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