Only a small amount of blood is taken during the test so you shouldn't feel any significant after-effects.
However, some people do feel dizzy and faint during and after the test. If this happens to you, tell the person carrying out the test so they can help you feel more comfortable. After a blood test, you may have a small bruised area on your skin where the needle went in. Occasionally, a larger area of bruising may appear. This can be because there was a lack of pressure at the site of the jab or the blood vessel was damaged by the needle.
Bruises can be painful but are usually harmless. However, tell your doctor if you frequently get bruises after having a blood test.