Society's version of Xanax, given to each member at age thirteen—it's for "calm[ing] us if we need calming" (5.82). Better not get addicted though, because you're limited to one a week. Most folks take them every so often, before a big test or the Match Banquet, for instance—but Cassia's always resisted because of something Grandfather told her when she first got hers:
"I wouldn't take that tablet, Cassia. Not for a report. And perhaps not ever. You are strong enough to go without it." (10.100)
For many, the green tablet is a source of strength when they are feeling weak. For Cassia's mom, for example, taking the green tablet allows her to be strong for her family and keep secrets when it's necessary for their survival. For Cassia though, being able to look at that tablet and know she has never taken it reminds her of her own inner strength—and it reminds us that she is different from other folks in the Society.