The state is planning to use three drugs - midazolam (a sedative), fentanyl (the high-potency opioid) and cisatracurium (a paralytic) - to execute Scott Dozier on Wednesday at 8 p.m. (11 p.m. ET).
Alvogen accused Nevada correction officials of illegally obtaining midazolam, as it is opposed to the use of any of its drugs in state-sponsored executions.
The order by Clark County District Court Judge Elizabeth Gonzalez delaying the execution marks the first time a pharmaceutical company has succeeded in halting an execution over legal or ethical concerns.
US drug manufacturers have increasingly refused to sell states any drugs that could be used in the procedure, after disturbing details emerged from a series of botched lethal executions across the nation.A drug company unsuccessfully sued Arkansas a year ago to ban the state from using one of its drugs in the procedure.
Scott Raymond Dozier's execution had been scheduled to take place Wednesday evening.
Dozier, 47, was convicted in 2007 of murdering and dismembering 22-year-old Jeremiah Miller five years earlier at a Las Vegas motel.
Dozier suspended any appeals of his conviction and sentence, making him one of about 10 percent of the 1,477 inmates who gave up appeals and were executed nationwide since 1977, according to the Death Penalty Information Center in Washington, D.C. Critics say he is seeking state-assisted suicide.
The delay leaves Nevada reassessing its options and stokes a debate about how the 31 states that have capital punishment can put anyone to death in an era when pharmaceutical companies ban their products for that use.
Alvogen and the state are scheduled to return to court on September 10 for another hearing in the case.
The Nevada department of corrections said it had no comment on the lawsuit.
An hour before the execution Wednesday, death penalty opponents plan to hold a vigil and rally outside the governor's mansion.
The document notes that midazolam (one of three drugs that were to be used in the lethal injection cocktail) "is not approved for use in such an application".
Fordham University law professor Deborah Denno said numerous drugs previously used for executions were manufactured in European countries that don't allow the death penalty. That combination of drugs has never been used in an execution.
In 2005, Dozier was sentenced to 22 years in prison for shooting to death another drug-trade associate, whose body was found in 2002 in a shallow grave outside Phoenix.
Rose said the rights group is examining why the distributor delivered the drug to the Nevada prison authorities even after it was publicly known they meant to use fentanyl to kill Dozier.
Nevada has not carried out an execution since 2006.
Midazolam, which the World Health Organization counts on its list of essential medicines, has nevertheless been implicated in a number of botched executions in other states.
The manufacturers of the other two unapproved drugs the state intends to use have also objected to Nevada's plans, though they're not formally a part of Alvogen's suit.
However, the legal challenge filed by Alvogen is only the second of its kind in the United States, said Robert Dunham, executive director of the Death Penalty Information Centre in Washington.
He reiterated his wishes to local paper the Reno Review Journal: "Life in prison isn't a life". But the state refused. Dozier has said repeatedly he wants to die. Fentanyl, a synthetic opioid well known for its role in the opioid epidemic, has "never been used in an execution before".
There was a limit to how much artwork and exercise a person can do in prison, Dozier said in court hearings and letters past year, according to ABC News in the US. Miller's headless torso was later found stuffed in a suitcase in a trash bin, media said. A witness testified Dozier used a sledgehammer to break Greene's limbs so the corpse would fit in a plastic storage container.
Death-penalty watchers have pointed to inconsistent results with midazolam since the 2014 executions of Dennis McGuire in OH and Josph Rudolph Wood III in Arizona.
Dozier, a former stripper and ice dealer, has said he doesn't care if the deadly combination of three drugs hurts, he just wants to die.