That wasn't really what Marek had meant. He'd intended that question as something he could do in the moment, right now, to make her feel better. But since she hadn't taken it that way, she was probably content with the way she was right now, and didn't need anything else.
"Veritaserum?" he said. "Or one of my other truth serums? That's easy to put in a blade. Or a pin, or a dart, or a ring - anything metal, really."
Designing the potion itself was always the hardest bit, really. Veritaserum wasn't a poison, but it was an already established potion, and Marek had yet to meet a potion he couldn't make if there were already instructions for it. Veritaserum was difficult, of course, but he'd had plenty of practice. Combining potions with metal was a special trick of his, and he needed no assistance in making that.