ST. LOUIS -- The third problematic execution in the U.S. in six months offers more evidence for the courts that lethal injection carries too many risks and amounts to cruel and unusual punishment, death-row lawyers and other opponents say.

Death-penalty opponents say an Arizona inmate who gasped for breath for more than 90 minutes in a nearly two-hour execution Wednesday showed that executions using different drugs and dosages are a callous trial-and-error process. The result: Every few months, a prisoner gasps, chokes and takes an unusually long time to die.

"These executions are experiments on human subjects," said Cheryl Pilate, an attorney for several Missouri death-row inmates. "The potential for things to go wrong is almost unlimited."

Lethal injection has been challenged in the courts many times, mostly without success. The biggest recent obstacle for death-penalty states has been obtaining lethal chemicals after major drugmakers, especially European ones, stopped selling drugs for use in executions. That forced states to find alternative drugs.

Adding to the concern from opponents is the fact that the drugs are mostly purchased from loosely regulated compounding pharmacies. Arizona, Texas, Florida and Missouri refuse to name the supplier and offer no details about how the drugs are tested or how executioners are trained.

The Supreme Court will probably face increasing pressure to examine how American executions play out, said Deborah Denno, a Fordham University School of Law death penalty expert.

"Every time this happens, it makes it far more difficult for a state corrections department to justify using a drug such as midazolam that's so consistently problematic, and to justify the secrecy," Denno said.

Some death-penalty opponents are focusing on midazolam, a sedative commonly given to people with seizures. It was first used in an execution in October in Florida.

This year, three of the 10 U.S. executions using the drug have gone wrong. The latest was Wednesday. Arizona Gov. Jan Brewer ordered a review of the state's execution protocol. Wood's lawyer demanded an independent investigation.

Governors in Ohio and Oklahoma ordered similar reviews after the bungled executions in those states earlier this year.

In January, Ohio inmate Dennis McGuire snorted and gasped for 26 minutes before dying. State corrections officials have said they do not believe McGuire suffered, but they increased the drug dosage "to allay any remaining concerns."

In April, Oklahoma inmate Clayton Lockett died of an apparent heart attack 43 minutes after his execution began. The state's prison's chief directed the executioner to stop administering the drugs when he learned there was a problem with the IV.

Both Arizona and Ohio used a two-drug protocol of midazolam and the painkiller hydromorphone. Oklahoma used a three-drug combination of midazolam, the muscle relaxant vecuronium bromide and potassium chloride, which stops the heart.

State protocols on how to use midazolam vary greatly. Some inject it as part of a two-drug method, others three. The amount of the drug given also varies. Ohio used 10 milligrams of midazolam in the McGuire execution. Oklahoma's protocol calls for 100 milligrams. Florida uses 500 milligrams.

"They don't know," Denno said. "We don't have experts on how to inject someone to death."

Texas and Missouri, two of the most active death penalty states, use the single drug pentobarbital. Still, death row lawyers say the same potential exists for problems to occur.

Pilate and James Rytting, a lawyer who represents several condemned inmates in Texas, plan to cite the Arizona execution in appeals for inmates awaiting execution.

"These agonizing and horrifying situations are going to happen," Rytting said.

Texas plans no changes based on what happened in Arizona, corrections spokesman Jason Clark said.

"The agency has used this protocol since 2012 and has carried out 33 executions without complication," Clark said.

Ohio corrections spokeswoman JoEllen Smith said the state is "always evaluating" policies to ensure executions "are carried out in a humane and lawful manner."

Louisiana Department of Corrections spokeswoman Pam Laborde said the department is "considering alternative methods of execution, including the most effective drugs and dosage levels" for lethal injections.