• excessive bleeding
• fainting or feeling light-headed
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

Pregnancy increases the need for folic acid.

Drugs that can decrease folate measurements include alcohol, aminosalicylic acid, birth control pills, estrogens, tetracyclines, ampicillin, chloramphenicol, erythromycin, methotrexate, penicillin, aminopterin, phenobarbital, phenytoin, and antimalarials.

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.