The common names for the plant that we call "buttercup" include Crowfoot, Gold Cup, Grenouillette, Meadow Buttercup. Magically it is associated with Love and used in rituals & ceremonies associated with marriage and Handfasting, engagements, and rituals involving commitments and sacred binding vows
The buttercup plant is poisonous to humans as well as to pets, cattle, horses, goats etc. Once dried it is no longer poisonous and buttercups in hay will not harm farm animals.
The plant produces protoanemonin, which is at its highest concentration at the flowering stage. It is quite unstable and drying of the plant leads to its polymerisation into a crystalline non-toxic anemonin. Protoanemonin is formed from the glycoside ranunculin when the plant is crushed.
This instability may explain why buttercups are not viewed with the same venom as Jacobaea vulgaris (syn. Senecio jacobaea), common ragwort, when it comes to livestock deaths. Since the toxins rapidly degrade, there is little risk of harm if Ranunculus species are included in conserved forage.
Ingestion produces inflammation of the mouth followed by abdominal pain. Ulceration of the mouth and damage to the digestive system follow. Diarrhoea occurs and urine can be bloody. Convulsions precede death. Protoanemonin is volatile and can be given off when handling the plant leading to eye and nasal irritation. http://www.thepoisongarden.co.uk/atoz/ranunculus_acris.htm
Treatment includes inducing vomiting and then washing out the system with an activated charcoal solution followed by supportive care.