Vials In most cases, air pressure inside the vial is similar to that of room air. In order to prevent the formation of a vacuum inside the vial (less pressure inside the vial than room air) the used should normalize pressure by first injecting into the vial a volume of air equal to the volume of fluid that is going to be withdrawn. If the drug is in powdered form, it has to be reconstituted. Inject the desired volume of sterile diluting solution (the diluent), such as sterile water for injection, into the vial containing the powdered drug.