Our experiences are just the opposite - James was poorly-sick after milbemax, but our vet said that it was just co-incidence. Next time he was given Milbemax, he was SO poorly - sickness, lethargy, dia-wotsits, not eating, not drinking etc. had to be re-hydrated...... again vet said it was unusual for milbemax to be the culprit. This time I wormed him with drontal - no problem what so ever, so the conclusion..? It depends on the dog, but be alert to possible problems!!!!!