to make sure blood comes into the syringe, to be sure you're in the vein and not just stuck into the body. many injections are intravenous, meaning they work best when put INTO the blood. some are subcutaneous (below skin, not in muscle) or intramuscular (into the muscle). Neither of those two require you to be in the vein, pull back is unnecessary (especially if you inject further from larger veins, say in the butt, or biceps muscle.
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