Renuyu was a dangerous and addictive chemical developed by the laboratories of Roland Daggett. The name is a pun on "Renew You".
Its exact composition is unknown; however, it is known to generate considerable addiction on the subject when it has been used for only a brief time. Part of the initial boom of Renuyu came, however, from the fact it allowed for temporary shapeshifting, presented as a miraculous form of skin conditioning; in reality, Renuyu loosened the skin cells so much it allowed to shape the facial features the user desired, as seen with Matt Hagen.
Also, it acted as a powerful drug, allowing for complete shapeshifting and mass regeneration - though to act thus it required saturation of the subject's (or in Hagen's case, victim's) cells with a massive overdose of Renuyu. However, a side effect was accelerated aging; the saturated cells would break down after some time and melt down the entire system, unless an untold number of chemical reactions were undertaken.
Roland Daggett saw Renuyu as his ticket for absolute success, though it was flawed and required testing. Though the flaws could not be corrected and Renuyu was still addictive and potentially dangerous, this did not deter Daggett, who tried to market it anyway. Seeing that he could not hope to do so with his own resources, he tried to introduce the cream into the market by acquiring Wayne Industries by insider trading and using its superior market to introduce the drug to Gotham households.
As part of his plan, Daggett approached actor Matt Hagen, who had been disfigured in a car accident, and offered him a cure with the Renuyu. Hagen accepted but quickly became addicted to the cream, which Daggett capitalized on by having Hagen participate in his scheme to acquire Wayne Enterprises in exchange for more. When Hagen botched an operation that could have led Daggett to prison, Daggett ordered Hagen killed, which his henchmen, Raymond Bell and Germs, carried out by forcing Hagen to ingest an entire canister of the Renuyu cream. Instead of killing him, the cream transformed him into Gotham's resident shapeshifter, Clayface. Meanwhile, with both Hagen and Lucius Fox apparently dead, Daggett attempted to advertise the Renuyu on Gotham Insider as a more potent face cream, but the Renuyu's true horrors where exposed when Clayface personally confronted Daggett on the show, while Batman analysed the cream and came to the same conclusion.
Daggett was arrested and held in custody, but even though he was freed due to lack of evidence, the Renuyu project was all but abandoned and the produced samples presumably destroyed. Despite this, the Renuyu's remnants continued to live on in Clayface, whose body slowly began to decay due to the Renuyu having saturated all of his cells. With Clayface's apparent destruction at the hands of the Flash and Hawkgirl many years later, it is possible that the Renuyu formula has now been completely eradicated, without a trace left in the world.