I have written a few comments over the last year about Equine intestinal parasites. I thought it was important to answer the question are Equine De-Wormers 100% effective, that is do they kill 100% of the intestinal parasites (Worms).
The answer is no.
No De-Wormers are 100% effective in killing all the worms.
That is why it is important to have a program for De-Worming that is frequent enough to not allow a buildup of intestinal worms.
Your Veterinarian can help you with the best program depending on how many horses you have, your location and problems in your area.