From 2018 NYT article:
Nebraska and Nevada hope to soon start using the opioid fentanyl as a sedative. Illegal use has made it a scourge of national death statistics, but medically it is an important painkiller and anesthetic. Defense lawyers in Nebraska have argued that fentanyl comes under a federal law that limits its distribution to lifesaving purposes, and that it is therefore illegal for a prison clinic to distribute it for an execution. A trial seeking information about the source of the fentanyl is scheduled for May 14.
Dr. Charles D. Blanke, who has studied data on physician-assisted dying, said it was not at all clear that nitrogen inhalation would bring a peaceful death. Dr. Blanke, a medical oncologist and professor at Oregon Health and Science University, said he had consulted colleagues in pulmonary medicine and anesthesiology, and they had concerns that carbon dioxide actually could build up and cause feelings of suffocation.
Veterinary experts generally do not recommend nitrogen or other inert gases for euthanizing mammals. Responses to the gas vary according to species, and in its 2013 guidelines, the World Society for the Protection of Animals said, “Current evidence indicates this method is unacceptable because animals may experience distressing side effects before loss of consciousness.”
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