All the Rage by F Paul Wilson

The provided text follows Jack, a "fixer" who operates outside the law, as he becomes entangled in a dangerous web involving Dr. Luc Monnet, a scientist, and Milos Dragovic, a ruthless mobster. Jack initially seeks to address the traumatizing actions of a drug-fueled individual named Butler, which leads him to discover a powerful and addictive new substance called Berzerk, linked to Monnet's research at GEM Pharma. Concurrently, Monnet is shown to be under the thumb of Dragovic, who forces him to produce the Loki molecule, which is the source of Berzerk, for a lucrative illegal market. The narrative also introduces other characters like Nadia, a colleague of Monnet, who seeks Jack's help to investigate the suspicious activities at GEM Pharma and the disappearance of her boyfriend, Doug, who was hacking into GEM's financial records. As Jack uncovers the connections between Monnet, Dragovic, and the drug, he orchestrates a series of retaliatory actions, culminating in a violent confrontation at Dragovic's party and an intense standoff at Monnet's lab, all while dealing with the unpredictable effects of Berzerk on himself and the re-emergence of a monstrous creature known as a rakosh.