Borgia plant

From Memory Alpha, the free Star Trek reference

The Borgia plant is a poisonous plant native to the planet M-113. It is listed as carbon group 3 vegetation, containing an alkaloid poison similar to the Earth nightshade family, with a chemical structure common to most M-class planets.

The M-113 creature taking the appearance of Nancy Crater blamed crewman Darnell's death on his ingestion of a Borgia plant. However, Spock discovered that the peculiar red mottling on Darnell's face was not among the symptoms of Borgia plant poison. Doctor McCoy subsequently assured Kirk that Darnell could not have died of alkaloid poisoning. (TOS: "The Man Trap")

The "nightshade" in question is probably the deadly nightshade, a plant that contains the toxic alkaloid atropine, as there are many harmless nightshade plants. The Borgia plant may take its name from the Borgia family, the most infamous of whom, Lucrezia Borgia, was rumored to own a hollow ring that she regularly used to poison drinks.