User blog comment:B214/Question and Answers/@comment-24098614-20131117052503/@comment-68.210.1.131-20131117082055

Unfortunately, Samael's curse is only the most effective against dragons, dragon-related beings - such as drakes and dragonoids - or anyone with a dragon type Sacred Gear. In the case of the latter, it dosen't matter whether it's a simple Twice Critical or the dragon type Longinus Boosted Gear and Divine Dividing. To Samael, if you're marked with a dragon, you'll suffer as such under his curse. Issei and Vali being cases in point, Issei even more so due to him having his left arm remade with dragonic flesh up until Volume 11. As for any other being - devil, angel, human with a non dragon Sacred Gear - it's just like a mild to significant poison, painful but not necessarily fatal depending on the target/victim. A possible exception could be made to this hypothesis depending on any possible children Issei might - or will - have with his harem due to his new body making him a devil/dragonoid hybrid.

As for Samael's curse against Super Devils such as Sirzechs and Rizveim, I sincerely doubt it will affect them at all. If Samael's Curse will affect any devil, the only one that comes to mind for myself is Euclid Lucifuge due to him possessing an "artificial" version of the Boosted Gear, and even THAT is a longshot in my honest opinion. Now if you want a poison that can be considered fatal - even deadly - to devils, that would have to come from the one creature that is also among the most dangerous in terms of Familiar candidates: the Nine Headed Hydra. Assuming the information on this wiki is correct, the Nine Headed Hydra possesses a poison that is to Devils what Samael's Curse is to dragons and dragon related cases. That being said, this does not necessarily apply to a Super Devil given that the poison has never been tested on a subspecies of Devil that is as rare as its members are strong. However, if the poison could be say, extracted and subsequently refined - or at least amplified - then I would say it could be effective. Till then, this will remain a hypothesis worth discussing. At least I think so anyways.