RL65 speeds up the natural healing factor of humans, allowing them to recover from seemingly mortal wounds in a matter of minutes, as well as apparently granting them a period of enhanced healing for some time after the drug was injected; it could even bring the dead back to life if they were injected quickly enough.
RL65 aggravated the amygdale, the part of the brain that manages aggression, in the process. The drug thereby caused episodes of rage in patients who took the drug (much like steroids), although this side-effect was apparently mitigated depending on the severity of the wound; Oliver Queen experienced mood swings and made irrational decisions for the better part of a day after he used the drug to heal a serious bullet-wound in his shoulder, but Lex Luthor was apparently unaffected in terms of his mental state when the drug was used to bring him back to life after he was shot with an arrow at close range.
The drug was created by Doctor Pamela Black based on research she carried out on mice, with her experiments being funded by both Queen Industries and Luthorcorp (Although Queen Industries was unaware of Luthorcorp's role), Lex Luthor arranging for RL65 to be given to residents of a halfway house he sponsored to test its effect on human tissue.
After his first encounter with Clark Kent, Oliver Queen began to use RL65 to heal himself as a means of coping with the kind of opponents he could face in a world where a being like Clark existed, first using the drug to heal a gunshot wound to the shoulder.
Shaken at the potential side-effects of the drug, Doctor Black burned her notes and the drugs, refusing to be part of the experiment if people were going to get hurt; Lex apparently arranged for her to be killed by one of the halfway house residents in revenge. Driven to irrationality by his use of the drug, Green Arrow almost killed Lex Luthor in a confrontation in his office, but Clark was able to save both of them. Realising what he had almost done, Oliver abandoned use of RL65, admitting that he started using the drug so that he could be like Clark, but recognised that Clark would never kill a man (Rage).
Lionel Luthor used RL65 as a cover story in his dealings with Lex Luthor. After Chloe Sullivan healed him from his gunshot wound, Lex woke up very confused about why he was completely fine, and Lionel gave him a possible explanation: That he had saved a dose of RL65 for himself, but decided to use it on Lex instead. (Fracture)