Ah, the mysterious red tablet—the one that Society's citizens are only allowed to take if a high-level Official tells them to. No one remembers ever taking it, but Cassia figures out why:
I know why we're going to take them. So we forget what happened to Ky, so we forget that the Enemy is winning the war in the Outer Provinces, that the villagers there are all dead. (29.70)
The red tablets are yet another manifestation of the Society's power and control over its citizens. From the way they are not allowed to know what the pill does, to how they can only take it when instructed, to its use as a device to control the minds of citizens and make them compliant, there's a whole lot of meaning packed into that tiny red pill.
And as such, there's also a whole lot of meaning packed into Cassia's refusal to take it. Cassia surreptitiously stomps hers into the grass instead of taking it as an act of defiance—she won't let Society control her mind. This is also an act of love for Ky, though—a refusal to forget him or the fate she's sent him to—and in this way, not taking the red pill is also a refusal to let Society govern her heart.