The link below is a table detailing the FDA approved anthelmintics. Not all of the anthelmintics listed are labeled for sheep. Check the FDA approved species column (4th from left) to make sure it is safe to use with sheep.
Powerpoint slides from a presentation by Dr. James Miller at the 2012 National Sheep Symposium
Internal parasites are the #1 health problem affecting small ruminants. Sheep and goats are more susceptible to internal parasites than other livestock, due to their grazing behavior and poor immunity.