Never had a problem but I'm not sure I would take the chance by adding more. Digitalis is a certain killer for sure but we do have the wild variety in the fields and nothing has died from it so far.
It's in the leaves and will give you a heart attach very easily. Back in the day when Robin Hood had his merry men half a dried leaf was used in heart conditions as a stimulant. Get it wrong and its good-bye Nottingham :-)