After riding the rollercoaster the next area is sideshow alley. As soon as you walk into it there's a newspaper article to read on the game booth in front of you, after reading it follow the blood stains to the left. After doing that, go to the middle area and there will be pills on a table. Take the pills, follow the area around. The gate will open to leave and then your achievement will pop.
You can see it being done in this video walkthrough of the game at 18:20