All species are extremely toxic and if horses eat too many flowers this can cause cardiac arrest.
There are different species of this plant:
the most popular Red Foxglove (purpurea), the Big-Flowered Foxglove (grandiflora), the Yellow Foxglove (lutea) and the Woolly Foxglove (lanata). Even if they differ from the color of the flower they all have one thing in common: all flowers offer the toxic substance Digitalisglycoside. Generally Foxglove grows in Middle- and Western Europe where it blooms from Mai to August. You can find it on areas of partial shade what means near forests or glades.
Foxglove is popular for its bell-shaped flowers which are responsible for the name of this beautiful plant. It grows up to 30-150 cm and does not become older than two years.
The percentage of the poisonous substance Digitalis is dependent on the respective time of the day and year. Therefore it varies from 0,1-1% and especially in the afternoon the percentage is higher than at other times of the day. The toxic dose for horses is 25 g of dried and 100-200 g of fresh flowers. Bloody diarrhea, staggering, drowsiness and cardiac arrhythmia are the typical symptoms for a poisoning. In worst case the heart stops to beat and the horses dies. For humans a dose of only two flowers has toxic effects!
In general physicians use Foxglove as medicine. The toxic substance Digitalis is one of the most important remedy for the stabilization of the heart rhythm.
In general one needs to control all places where horses are without supervision (e.g. pastures) if there are any poisonous plants.