From memory, there are only so many substances that can cause that level of incapacitation
Carbon monoxide and/or hypoxia for example can cause confusion and other described symptoms:
https://www.cigna.com/healthwellness...lertness-confu
https://cfpub2.epa.gov/ncea/risk/rec...TOKEN=19447862
The health effects associated with exposure to CO range from the more subtle cardiovascular and neuro-behavioral effects at low concentrations
After landing some passengers spoke about CO:
“It was pretty scary,” he said, adding they were told there were some technical problems with the plane. “The cabin crew on the top deck weren’t feeling very good because it was something to do with some carbon monoxide or something, that’s what [the firefighters] were talking about,” he said.
British Airways flight forced to land in Vancouver after crew members fall ill | Globalnews.ca