Kenji never imagined he'd actually get drugged by the Empress.
There were plenty of people in the palace who wanted him dead, but ironically, that same palace was exactly why such methods—like poisoning—were rarely used. As a top official, he almost never stayed in the palace. If he suddenly "fell ill" in the palace, anyone with half a brain would suspect the royal family's involvement. He had plenty of followers in court; if word got out he was poisoned, there would be chaos.
That was why he'd let his guard down a bit. After all, who'd expect someone to actually dare drug an external minister inside the palace? If empress dowager's plot was revealed, the court would explode.
Still, when he picked up the tea, Kenji noticed something was off about the taste. The difference was incredibly subtle, but he could sense something wasn't right, so he only took a small sip. He quickly realized there was something in the tea.