The choice of poison is extremely suggestive. Why choose an obscure poison that no one has heard of? Why not, more simply, arsenic, or strychnine, or even cyanide? Why does Anthy use the example of La Cantarella?
La Cantarella, the poison (whose formula is unknown today) that Anthy talks about, was rumored to have been used by the (older) brother and (younger) sister, Cesare and Lucrezia Borgia, to murder some of their enemies. Despite the vile crimes attributed to Lucrezia by historians over the past five centuries, more recent scholars have found evidence indicating that she was more likely a very young and (mostly) innocent pawn of the schemes of her brother and their father, Pope Alexander VI. She was betrothed twice, married three times, and bore a child that was recognized in two papal bulls as the "Infans Romanus," first as Cesare's child, and then as Alexander's child. So it seems clear that she was a victim of incest.
Her first marriage ended with Alexander driving her husband out of the country, while the husband made accusations of incest between Lucrezia and her father and brother. The husband eventually capitulated to Alexander's demands and signed an agreement that the marriage had never been consummated (saying that he was impotent, apparently). Her second marriage, begun to forge an important alliance for her brother, ended with her husband being strangled by her brother's servant. Her third marriage lasted and, after her father's death, allowed her to become a renowned patron of the arts.
One can take some of this history and draw any number of parallels between it and the Anthy/Akio relationship:
Utena's response? It contained a degree of denial of the warning and potential threat --- she doesn't know the history, perhaps, or just doesn't want to see Akio as evil quite yet. It was, so far as Utena could understand and recognize the warning, a declaration of trust in Anthy -- which is probably what resulted in Anthy's later suicide attempt, the only option Anthy could perceive to enable her to deserve that trust.
The reciprocity of poisoning? Utena recognizing the suggestion of poisoning as an admission by Anthy that she had hurt her, and responding in kind (creating the metaphor of drinking secretly poisoned tea and eating secretly poisoned cookies for their subtly poisoned everyday life). This last is what enabled them to go on and discuss -- and promise -- their meeting in ten years, to drink tea and eat cookies just like this.
Could that be a promise of bringing the secrets that poisoned their lives out into the open to be digested in later years, a process of catharsis? Who knows?
Certainly I may be reading too much into a very short exchange, but I think it's clear that one of the great accomplishments of Utena is its ability to pack a great deal of visceral impact into very short bursts of imagery or dialogue.
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