Possible spoiler about Yoshiko

I assumed it was poison too, thinking it was kind of a plot hole for Iwabuchi to be so naive in taking it. But when Yoshiko woke up, it was obvious that Kurosawa was tricking the audience, playing with their expectations.

I understood that it was a sleeping powder. Iwabuchi's boss had given him an ultimatum - find the saboteur, kill him, and put an end to the plot, or commit suicide. He sent him the powder to bring the point home; Iwabuchi, the perfect company man, practically grovels thanking the boss for it, even saying how thoughtful he had been to tell him the exact dosage. It's left unclear whether the dosage is the amount that will help him sleep, or kill him. In the same scene the script alludes to the fact that Iwabuchi is having trouble sleeping. I believe he was mixing himself a mild sleeping potion, intending to rest and then continue his pursuit of Nishi, when he saw Yoshiko arrive home and realized where she had been.

The theme of insomnia is interesting. A number of characters complain of it or allude to being up most of the night. Tetsuo and Yoshiki repeatedly refer to Nishi's nocturnal ways, and at the end of the film he confesses to not having been able to sleep. But some of the bad characters, including Iwabuchi, Shirai, and Wada, also have trouble sleeping because of their guilt over their sins. Kurowsawa could be inferring that they still have a spark of conscience or goodness in them - or that he intended the title as ironic, making Nishi as culpable as the rest.