Ohio executioners in 2006 needed more than an hour to put Joseph Clark to death because of trouble with his veins.
Later that year, it took Florida killer Angel Diaz 36 minutes to die, with an autopsy showing that the needles that were used to send the lethal drugs into his veins had penetrated his muscles.
In 2009, Ohio abandoned an execution attempt after Romell Broom was pricked 18 times with needles. Broom remains on death row, challenging the state's right to try again.
In January, an Ohio inmate snorted and gasped during the 26 minutes it took him to die. The state said this week it doesn't believe Dennis McGuire suffered, but it also announced it would increase the drug dosages "to allay any remaining concerns."
Also in January, Oklahoma death row inmate Michael Lee Wilson said during his execution: "I feel my whole body burning."
The use of custom-mixed, or compounded, drugs by Georgia, Missouri, Ohio, Oklahoma, South Dakota and Texas has opened a new line of attack for defense attorneys, who have demanded to know the identities of the drug suppliers. They have argued that deficiencies in the purity and potency of the drugs could cause a slow and torturous death.
Bolstering this argument: Compounding pharmacies are not regulated by the federal government, and a 2012 meningitis outbreak that killed 64 people was linked to contaminated drugs from one such business in Massachusetts.
But courts so far have rejected those arguments in a number of states.
Legal issues aside, one problem with the cruel-and-unusual argument is the difficulty in measuring pain in unconscious people, whether surgical patients or death row inmates.
Doctors can rely on heart monitors or blood pressure machines to measure reaction in sedated patients, but those methods are not foolproof. Such machines aren't used in death chambers.
Associated Press writers Jim Salter in St. Louis and Michael Graczyk in Houston contributed to this report.