The risks associated with drawing blood are minimal but may include bleeding from the puncture site, feeling faint or lightheaded after the blood is drawn, or blood accumulating under the puncture site (hematoma).
Dizziness. Changes in blood pressure and in how your blood vessels react can make you feel a bit lightheaded if you stand up quickly or remain standing very long.
In general, syncope is defined by a brief loss of consciousness (fainting) or by dimmed vision and feeling uncoordinated, confused, and lightheaded.
In this way, it is not uncommon to feel slightly dizzy or lightheaded, either before your heart begins to pound or after.
You may start sweating as nausea overcomes you or feel dizzy, lightheaded and even faint.