The usual blister plaster was a dressing applied over a powder made up of crushed Spanish flies (cantharides
), an irritant which causes a blister to rise on the underlying skin.
Hollenkemp, an 1852 case, the court let stand a punitive damages award against a local drug store where the store's agent had accidentally mixed cantharides
in the plaintiff's medicine, making him ill.
, or powdered Spanish Fly (a kind of beetle), powdered bluebottles and groundsel were regarded as love philtres but were far from safe.