This picture tells the whole story. Don't EVER resuse your needles and always use a different needle to draw and inject. If using a slin pin, it is beneficial to use the backloading method which invloves removing the plunger and shooting the gear into the syringe from a separate needle. We already have more scar tissue than we want...this is how you avoid the buildup. Needles are cheap...especially in comparison to your health.