Deadly Night Shade causes a person to fall into a deep slumber after consuming it. It apparently also induces symptoms similar to a hangover a day after ingesting it, as Finkelstein was seen with an icebag on his cranium after grounding Sally for her earlier stund, and when hearing the doorbell alarm, proceeds to whine about his head in agony.
This ingredient is used by Sally to put Dr. Finkelstein to sleep after she poisoned his Worm's Wort Soup. She then attempted to hide the odor of the Night Shade with Frog's Breath. When the Frog's Breath repelled the scent of the Night Shade, it made the soup bitter so Sally used Worm's Wort to freshen up the soup and make the soup taste better.
Deadly Night Shade appears as one of Sally's primary weapons in Disney Heroes: Battle Mode. It also appears as a badge and avatar icon. In the game, Sally throws a flask of Deadly Night Shade as one of her abilities, whereas one of the abilities is titled "Deadly Nightshade," which allows Sally to stash-poison her enemies.