PG67A/W is a special, Progenitor-based serum used to keep the virus within Albert Wesker's body stable and stop him from losing his strength. The "PG" may be an acronym for "Progenitor".
It was administered to him regularly by Excella Gionne. The dosage given must be extremely precise and taken at certain times, otherwise it can act as a poison.
During Chapter 6-3, Jill instructs Chris and Sheva (in a cutscene) to inject the serum into Albert Wesker so that it should act "like a poison". During the second fight with Wesker in the Hangar, the player's mission is to inject the serum on him, causing an overdose.