Deworming Schedules:

Frequency:
In areas of high concentrations of horses (many horses in a small area) the horses should be dewormed every 2 months. In areas of low concentration (few horses in large area) the horses should be dewormed at least every 3 months.
Rotation:
Rotations of dewormers helps to avoid resistance to the dewormers and utilizes the uniqueness of each dewormer (some are more effective against certain parasites).

Tapeworms:
Tapeworms appear to be an emerging problem due to the sometimes exclusive use of ivermectin. A double dose of strongid used to be the only dewormer to kill tapeworms. There are two new products on the market in 2003 that contain praziquantel, which specifically targets tapeworms. The new products are Zimectrin Gold, made by Merial and Equimax, manufactured by Pfizer. Tapeworms can cause diarrhea or colic.
Quest (Moxidectin): Quest was first released in February of 1998. There have been reports of Quest causing abortion seizures and other problems. The dosage of quest per body weight must be accurate as an overdose of this drug can cause problems.The safety factor of quest is about 4x where as the safety factor of Ivermectin is said to be 40x.

A good deworming program is the following: