The Solanacea are a variety of similar plants, many of them are used as food, including tomatoes, potatoes, bell peppers and chilli peppers. Tomatoes, and many solanaceaes, are rich in vitamin C.
Among them, there is also unedible plants with mild neurotoxins, like the tobacco, which contains nicotine.
However, some solanaceae are very poisonous, like the belladonna. I will quote the relevant section from wikipedia:
Belladonna is one of the most toxic plants known, and its use by mouth increases risk in numerous clinical conditions, such as complications of pregnancy, cardiovascular diseases, gastrointestinal disorders, and psychiatric disorders, among others. All parts of the plant contain tropane alkaloids. Roots have up to 1.3%, leaves 1.2%, stalks 0.65%, flowers 0.6%, ripe berries 0.7%, and seeds 0.4% tropane alkaloids; leaves reach maximal alkaloid content when the plant is budding and flowering, roots are most poisonous in the end of the plant's vegetation period. Belladonna nectar is transformed by bees into honey that also contains tropane alkaloids. The berries pose the greatest danger to children because they look attractive and have a somewhat sweet taste. The root of the plant is generally the most toxic part, though this can vary from one specimen to another.
The active agents in belladonna, atropine, hyoscine (scopolamine), and hyoscyamine, have anticholinergic properties. The symptoms of belladonna poisoning include dilated pupils, sensitivity to light, blurred vision, tachycardia, loss of balance, staggering, headache, rash, flushing, severely dry mouth and throat, slurred speech, urinary retention, constipation, confusion, hallucinations, delirium, and convulsions.
So, what you need is a species of solanaceae that:
Is common and abundant in the area.
Has some of belladonna's toxins, but on a lower concentration, so people who consume it become intoxicated, but do not die.
Be the sole relevant source of vitamin C in the area, so its inhabitants have to consume it periodically in order to avoid scurvy.
The plant also contains nicotine, so people that consumes it become addicted and don't try to endure scurvy or trade for some substitute.