It's hard to say whether you should add the Lizard Gland to the brew or not in a choice-heavy game like The Blood of Dawnwalker. Usually, your choices have consequences, but in the Bittersweet Toast Quest, that's not the biggest choice you make. The more important choice is whether you let Iulian drink the brew or not, as this is going to affect your fight against Ambrus later.
The following guide contains spoilers for The Blood of Dawnwalker.
In my experience, after trying all of this Quest's endings, no, you shouldn't add the Lizard Gland to Vera's brew. That choice will not change the outcome, but it may affect your conscience.
Adding the Lizard Gland to Vera's brew will save Iulian's life later in the Gilded Gauntlet Quest in The Blood of Dawnwalker. This means Iulian will die if you don't add it, something he explains himself during the Bittersweet Toast Quest. Knowing this gives the choice of adding the Lizard Gland to the brew some serious moral weight. Gameplay-wise, it doesn't matter whether you add the Lizard Gland or not, as Iulian will make it to Ambrus's duel and successfully poison Ambrus. After that, you won't interact with Iulian again or have any more Quests with him.
During the Bittersweet Toast Quest, while Iulian is mourning Vera's death, he says it's because of that idiotic idea of hers. If you ask Iulian what idea, he admits he is the blood cow; he is the one Ambrus will feed on before the duel.
Iulian was going to poison and sacrifice himself. Vera insisted there was a way for him to survive and died trying to get a Lizard Gland. Iulian explains that Ambrus has been feeding on him for months. Ambrus makes Iulian beg him like a dog to feed on him in exchange for letting him pretend he's a powerful and noble man among his people. This is the main reason Iulian doesn't want to survive, as he only wants to be the reason for Ambrus's downfall.
During the Bittersweet Toast Quest, if you keep following the trails, you can find the monster that killed Vera and defeat it to get the Lizard Gland. That will unlock a new dialogue option where you tell Iulian you found the Lizard Gland.
Iulian insists on not adding it to the brew, as that was not his plan with Vera or you. I personally had hoped that if we defeated the Vrakhir, Iulian would feel better and gain his self-respect back. But that wasn't the case, as after defeating Ambrus, Iulian was surprised he was still alive and asked Coen if he had done something. At this point, you can tell the truth or keep lying that you had nothing to do with him staying alive. In both cases, you'll see a very broken Iulian who can't accept that he's still alive. It's even worse if you admit you added the Lizard Gland. Iulian will be mad and basically tell Coen that he doesn't care about anyone but himself. You got your perfect resolution; enjoy your clear conscience. It's all about you after all. That still didn't feel like the right ending for this entire Quest for me.
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