Common But Deadly Plants 4. Castor Beans: Castor oil has long been used to aid digestion. Though it has a foul taste & consistency, it is not harmful when ingested, yet one castor bean used to make castor oil can be fatal to an adult in minutes. That's because while making oil a lethal compound called ricin is removed, making it safe to consume. If you survive consumption of the bean you can suffer from week-long symptoms like nausea, emesis, diarrhea, tachycardia, hypotension & seizures.