There are nine free Zombrex doses. Only seven are required to complete everything in the game.
1.) Royal Flush Plaza - Case related. It is in Big Roy’s Mart.
2.) Americana Casino - Go to the second floor of the BBQ restaurant and take a right at the fork. Jump up on the balcony and over to the closest light. Proceed across the rest of the lights.
3.) Slot Ranch Casino - Backstage behind the curtain. Climb up a stack of boxes and speakers in the far back. Jump onto a smaller stack of boxes and speakers.
4.) Underground Access - Directly under the Atlantica Casino. The underground path overlaps the upper edge of the casino. The Zombrex is near an access ladder on a raised walkway.
5.) Code Blue (mission) - Save the paramedic (Sven).
6.) Yucatan Casino - On top of the tallest slot machine display in the middle.
7.) Royal Flush Plaza - Defeat the Postal Worker (Mission).
8.) Silver Strip Diner – Survivor: Richard (Hunger Pains). - Bring him food and rescue him.
9.) Safe house – Day 3 around 9 PM. - If you saved Sven, talk to him in the safe house and bring him whiskey or vodka for use as a disinfectant. In return, Sven will provide you with one last dose of Zombrex.
Credit to Nogarda for locations 1, 2, 3, 4, 7, and 8. Credit to andylee999 for locations 5 and 6. Credit to VakuA for location 9.
I feel my blood rushing through my body. Yes, this is it. The feeling - it's coming back to me: the sensation of killing.