Vasovagal syncope is where mental stress cause physical symptoms in this case fainting. Vasovagal syncope is the most common type of fainting. The basics behind it is that the nerve that controls the cardiopulmonary tract (vagus nerve, aka cranial nerve X) is over stimulated and causes an inhibitory effect on the heart, lungs, and visceral organs. This causes a drop in blood pressure, heart rate, breath rate, ect.
This is not caused by a sudden drop in blood glucose levels in most cases. Though it is unlikely that pushing a needle into the skin would cause a sudden and rapid drop in blood glucose. Individuals that are already in a hypoglycemic state may increase the risk of passing out because of mental stresses.
Passing out while giving blood is a prime example that there is no sudden drop in blood glucose levels. A condition of hypovolemic state after giving blood is the most common complication of giving blood other than bruising. This is why a donor is giving a sugary or salty drink to help fluid balance post donation.
Tips to prevent passing out while injecting are as follows.
1. Calm down and control your breathing.
2. Sing a song or look away after the initial jab so that your mind is else where
3. Have a friend perform the injection.
Links to the rest of the series:
The difference between normal injection pain and infection
http://forums.steroid.com/showthread.php?t=358617
Injection pain: Why it happens and how to stop it
http://forums.steroid.com/showthread.php?p=4177202
How to filter your gear (With Pictures)
http://forums.steroid.com/showthread.php?t=358623