Ricin
A dose the size of a few grains of table salt can kill an adult human. The median lethal dose (LD50) of ricin is around 22 micrograms per kilogram (1.78 mg for an average adult, around  of a standard aspirin tablet/0.4 g gross) in humans if injected  or inhaled. Oral exposure to ricin is far less toxic and a lethal dose can be up to 30–40 milligrams per kilogram.