The severity of plant poisoning depends on the type of plant consumed, the amount ingested, and individual sensitivity. Some common plants that are toxic to humans include:

1. Castor bean (Ricinus communis): The seeds contain ricin, a deadly toxin that can cause severe vomiting, diarrhea, and abdominal pain.
2. Oleander (Nerium oleander): All parts of the plant are toxic, and ingestion can cause cardiac arrhythmias, seizures, and death.
3. Rhododendron (Rhododendron spp.): The leaves and flowers contain grayanotoxins, which can cause vomiting, diarrhea, and difficulty breathing.
4. Taxus (Taxus spp.): The leaves, seeds, and stems of yew (Taxus baccata) and Pacific yew (Taxus brevifolia) contain a toxin called taxine, which can cause vomiting, diarrhea, and cardiac problems.
5. Aconitum (Aconitum spp.): Also known as monkshood or wolf's bane, all parts of the plant are toxic and can cause nausea, vomiting, and abdominal pain.
6. Belladonna (Atropa belladonna): The leaves, stems, and roots contain atropine, which can cause dilated pupils, flushed skin, and difficulty urinating.
7. Deadly nightshade (Atropa belladonna): All parts of the plant are toxic and can cause nausea, vomiting, and abdominal pain.
8. Hemlock (Conium maculatum): The leaves and seeds contain coniine and gamma-coniceine, which can cause muscle weakness, paralysis, and respiratory failure.
9. Lantana (Lantana camara): The berries are toxic and can cause vomiting, diarrhea, and abdominal pain.
10. Oleander (Nerium oleander): All parts of the plant are toxic and can cause nausea, vomiting, and abdominal pain.
11. Castor bean (Ricinus communis): The seeds are particularly toxic and can cause vomiting, diarrhea, and abdominal pain.
12. Rhododendron (Rhododendron spp.): The leaves, stems, and flowers contain grayanotoxins, which can cause nausea, vomiting, and difficulty breathing.
13. Yew (Taxus spp.): The leaves, seeds, and stems of yew contain a toxin called taxine, which can cause vomiting, diarrhea, and cardiac problems.

It is important to note that while these plants are toxic, they can also be safely used in herbal remedies when prepared and administered properly under the guidance of a qualified practitioner. It is always best to consult with a medical professional before using any herbal remedy, especially if you have a medical condition or are pregnant or breastfeeding.