In Episode 3 of the anime Ryuuki mentions that the dose of poison meant for Shuurei was not meant to kill her. Given the most likely culprit, then why bother with poison if they didn't want to kill her? The answer comes in a short scene earlier in the episode when two ladies of the court gossip about Ryuuki sharing Shuurei's bed. Of course the poisoner's intent wasn't to kill her, it was to cause her to miscarry!
