Risks

There is very little risk associated with taking blood from a vein in the arm, although there is a slight risk of infection anytime the skin is broken. 

A bruise may develop in the area where the needle was inserted. Sometimes, a hematoma can develop at the puncture site. A hematoma is a large accumulation of blood underneath the skin, whereas a bruise is a small accumulation of blood underneath the skin. If you think you may have a hematoma and it appears to be growing larger, put manual pressure over the site, which should stop the bleeding after several minutes. If it does not stop, you should contact your doctor. 

Continued or excessive bleeding is a possible risk, especially for those people with bleeding disorders. People who take blood-thinning medicines are at more of a risk of developing a hematoma.

Some people may feel lightheaded after having a blood sample taken. Lying down or having something to eat or drink can usually treat lightheadedness. People who feel lightheaded may faint, but this is rare.