Spartium junceum (Fabaceae) commonly known as 'Spanish broom' is toxic to humans and can cause discomfort and irritation but is not life threatening. The whole plant is toxic including flowers. The toxicity of the plant is due to presence of 'spartine' and 'cytisine'.
"Aconitum Napellus, or Aconite, is from the plant Monkshood. This plant has been used in the past as a poison for dipping arrows. Aconite's poison acts very quickly on the systems of the body. Treating with the homeopathic principle "similar with similar", Aconite is used in treating sudden symptoms. Homeopathy is unique in turning poisonous and toxic substances into safe medicines. Vibrational medicine without side effects".
Thus toxic substances can also have medicinal value.