Marriage Toxin manga volume 7 features story by Joumyaku and art by Mizuki Yoda.

It's Poison Master versus Poison Master as Gero is forced to face off against his cousin Teruaki! After learning about Gero's frivolous marriage plans, Teruaki targets Kinosaki in order to help Gero regain his focus as heir to the Poison Clan - but Teruaki's tolerance to poison may be even higher than Gero's own. Can he stop Teruaki and save Kinosaki before it's too late?