My 5 pound dog took a 20 mg furosemide will it hurt him
Probabilities are that it will not hurt. This is because 20 mg of furosemide is the lowest dosage that should be taken. You can see a vet for more details.
That is a dierettic (misspelled I know, but it means water pill). I suggest you take your dog to a vet asap. They are not supposed to take things like that unless prescribed by a vet. Then when you can, lock up all meds so it does not happen again.