Why Oklahoma tried to execute a man with a secret, untested mix of chemicals

Vox explains Why Oklahoma tried to execute a man with a secret, untested mix of chemicals.

"Oklahoma was using the experimental formula because pharmaceutical companies increasingly refuse to supply "safe" lethal injection chemicals. That's left capital punishment states to choose between executing inmates under dangerous conditions or not executing them at all. Many states have chosen to go ahead, and some have adopted secrecy laws that shield the chemical compounds used for the executions."

"The key chemical in lethal injections is sodium thiopental, originally invented as an anesthetic. But US manufacturers of the drug have been increasingly refusing to sell it, either out of opposition to the death penalty or concern about association with executions. In 2011, the last US supplier, a company called Hospira, stopped making it. Later that year, the European Union announced an export ban on sodium thiopental, in pursuit of its official goal of "universal abolition" of the death penalty"

I wonder why they don't use the same medications that are used in Oregon and Washington for right-to-die prescriptions. Maybe it's the same drug and it's sale is limited based on intended usage?