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