Chemical Dependency is a mission in Dead Rising 2. It takes place in the Safe House in the early morning of the third day. It involves giving Zombrex to a bitten survivor, Jared Davis. Note that you will need to start and complete the mission Once Bitten to start Chemical Dependency.
Since Jared was bitten before being saved, he requires Zombrex to keep his well-being. He will be in the lounge, isolated from the other survivors, waiting for the medicine. When Chuck gives him the Zombrex, he will thank him for saving his life again.
If Jared does not get the Zombrex in time, he will be shot down by Raymond Sullivan to prevent him from turning into a zombie and infecting other survivors.