Deadly Nightshades were mentioned once in the series, in A Hero's Guide to Deadly Dragons.
Hiccup is racking his brain trying to remember how to deal with Poisonous Piffleworms.
- "Was it that they couldn't stand the color yellow? No, that was Deadly Nightshades . . ."
It is entirely possible the author meant Toxic Nightshades. However, Toxic Nightshades were mentioned a few pages back, in that they are dealt with by blowing in their eyes. In fact, Toxic Nightshades are yellow in color, so being put off by their own coloration doesn't make sense. Similarly, Deadly Nadders are also mentioned, and they are dealt with by rubbing their tummies with the end of a nettle.
Because Deadly Nightshades are mentioned along with poisonous dragons, as well as the nightshade plant is poisonous, we can assume the Deadly Nightshade is poisonous, too.
- A Hero's Guide to Deadly Dragons (Book 6), Cressida Cowell. 2007.