Dead 'til Dawn by Gerald Dean Rice
Two friends who have drifted apart go on an unintended journey when they take a drug that technically turns them into zombies for 72 hours. But near immortal cultists intent on journeying to the other side will stop at nothing to get the 'Bloom' back, even if it means taking it from their undead bodies. As the two men run for their unlives they test the bonds of friendship and learn the whole universe is depending on them. So we're probably screwed.