Let’s see - the case is over 2000 years old, there is no body or mummified remains or even bones left to examine, the man died in Babylon, a city which no longer exists, and the only witnesses apparently wrote varying accounts of his death in ancient languages on papyrus. Might as well consult a Ouija board - it’s all just speculation at this point.
Restina came about, I think, because the Persians added turpentine to wine in the hopes of poisoning the Greeks, and the Greeks liked it so much they asked for the recipe.
Accidental poisoning in the quest for a better kick is certainly a possibility. Then again, so was deliberately poisoning the conqueror - especially if promising a better high.