He'd never poison Darcy. In fact, his eyes widened with alarm as she coughed and made noises, and he actually wondered if someone else had poisoned the alcohol he had used. He was torn between daring to give her a pat on the back or shaking her by the shoulders, and finding out who needed to be ripped apart as soon as he could ascertain who was responsible....
When she recovered, he knew it was a joke. And a fine one at that, enough that Loki started laughing.
"Nicely done!" he commended her. "I have played a fine joke of my own, which should take but a small while to come to fruition. Avoid the beverage in yonder vessel."
Loki put a finger to his lips to his partner in pranking crimes, like it was their little secret. He then pointed over toward the punch bowl, which a couple from down the hall was already trying out.
"Shh. Not poisoned or enchanted in any way, and is rendered harmless enough for Midgardian constitutions. It is, however, particularly potent."