What is the primary reason to use a vented needle during compounding?

Using a vented needle during compounding is primarily intended to allow diluent to enter a vial without creating a vacuum. When a diluent is withdrawn from a container, it creates negative pressure inside the vial. If air cannot enter to equalize this pressure, it can be difficult to draw the liquid out, and it may also introduce issues with sterility if attempts to forcibly extract the liquid are made.

A vented needle has a small opening that allows air to flow into the vial as the liquid is drawn out, thus preventing the formation of a vacuum and facilitating a smoother and more efficient extraction process. This is particularly important in maintaining the integrity of the product and ensuring that the correct amount of liquid can be obtained without compromising sterility or creating an unsafe compounding environment.