I've seen many examples in ancient literature that mention the dangers of drinking "neat" wine (ie. not diluted with water), specifically either insanity or death. Here are a few well-known examples:
"...There he (Alexander the Great) drank much unmixed wine in commemoration of the death of Heracles, and finally, filling a huge beaker, downed it at a gulp. Instantly he shrieked aloud as if smitten by a violent blow and was conducted by his Friends, who led him by the hand back to his apartments. His chamberlains put him to bed and attended him closely, but the pain increased and the physicians were summoned... This was how he died after a reign of twelve years and seven months."
"There was still a hope of saving the life of Brennus, so far as his wounds were concerned; but, they say, partly because he feared his fellow-countrymen, and still more because he was conscience-stricken at the calamities he had brought on Greece, he took his own life by drinking neat wine."
"...the Spartans themselves say that Cleomenes' madness arose from no divine agent, but that by consorting with Scythians he became a drinker of neat wine, and the madness came from this."
This seems pretty strange, since people today drink plain wine all the time without dying or going crazy. Was this belief somehow based in fact? For instance, was wine much stronger back then, or did the production of it include trace amounts of poisons? And if not, where might have this belief have come from?
This is my first time posting so I do apologize if this post isn't acceptable. The reason they believed this is because Roman and Greek wines were a lot thicker and had a higher alcohol content. The "madness" at least by the time of Galen was to come from drinking immodestly. There are stories of Spartans getting helots drunk on unmixed wine so their sons could see what a drunkard looked like. As an adendum you can Google ancient Greek drinking vessels that had eyes painted on the bottom to serve as a warning that people were watching.