Morphine, xylazine. It's extremely fast-acting

Twenty-eight years since the rage virus escaped a biological weapons laboratory, now, still in a ruthlessly enforced quarantine, some have found ways to exist amidst the infected. One such group lives on a small island connected to the mainland by a single, heavily-defended causeway. When one member departs on a mission into the dark heart of the mainland, he discovers secrets, wonders, and horrors that have mutated not only the infected but other survivors as well.