After reading many of the posts here, regarding various Hartz products, I recall noticing my one cat twitching his back several times over the course of 2 or 3 days, following his last treatment of Hartz UltraGuard Pro/Plus. The other two cats seemed fine… though none of them was happy about the treatment! Anyhoo, I’ve done some research and am going to give the brewer’s yeast/garlic tablet treatment a try. I went to www.onlynaturalpet.com where there is a plethora of natural (no chemical) products, worth giving a try. I’ll keep ya posted on the results. Take care, all.