As the effectiveness of the dewormer decreases, it provides less and less benefit, and once it falls to <50%, it is no longer useful as a sole treatment. Given this situation, what is the best approach for using dewormers? Contrary to popular belief, rotating between dewormers will not prevent resistance from worsening, and is no longer recommended. Rather, dewormers should be used together at the same time in combination.
Research done in New Zealand has convincingly shown that the best approach is to use several different dewormers all at one time as a combination treatment.
Read full article by Dr. Ray Kaplan