Lawyers for Richard Cooey argue in a federal lawsuit that Cooey - 5-feet-7 and 267 pounds - had poor veins when he faced execution five years ago and the problem has been worsened by weight gain.
The lawsuit, filed Friday in federal court, also says prison officials have had difficulty drawing blood from Cooey for medical procedures.
Cooey, 41, is sentenced to die for raping and murdering two young women in 1986. His execution is scheduled for Oct. 14.
His attorneys say a drug he is taking for migraine headaches could affect the execution process. The drug Topamax, a type of seizure medication, may have created a resistance to thiopental, the drug used to put inmates to sleep before two other lethal drugs are administered, Dr. Mark Heath, a physician hired by the Ohio Public Defender's Office, said in documents filed with the court.
Heath says Cooey's weight, combined with the potential drug resistance, increases the risk he would not be properly anesthetized.
"All of the experts agree if the first drug doesn't work, the execution is going to be excruciating," Cooey's public defender, Kelly Culshaw Schneider, said Monday.
Prison system spokeswoman Andrea Carson and Jim Gravelle, a spokesman for the Ohio Attorney General's Office, both said Monday they hadn't seen the lawsuit and couldn't comment.
Last year, Carson cited the obesity of condemned inmate Christopher Newton as one of the reasons prison officials had difficulty accessing his veins before his execution. Newton was 6 feet tall and weighed 265 pounds.
Two years ago, convicted killer Jeffrey Lundgren was put to death after a federal appeals court rejected his claim that he was at greater risk of experiencing pain and suffering because he was overweight and diabetic.