"Dahlia's first potion effectively made me invisible to the Infected, but it only lasted for 15 seconds, and severely distorted my motor coordination. It's as debilitating as it is useful. Perhaps a stronger potion would have fewer effects-or at least last a bit longer."
All you need is kill a goon and loot him, then come back to Dahlia for a potion and blueprint. The potion has a side effect of slowing you down by 99%.
Note: you must drink Dahlia's potion from the previous quest in order for this quest to become available.
