I have NEVER bought flea collars for my dogs, so when I read up on this one, I figured it'd be the best, So, I bought 2 small and 1 large. NEITHER, did what they claimed to do (Great selling pitch tho 🙄) After 2.5 weeks one of my lil guys developed and AWFUL rash (bumpy,scaley dark red knodules, see photos) so I took the other collars OFF my other dogs immediately! Ran a flea comb through them, I got from my vet, and ALL 3 STILL Had LIVE fleas on them. DO NOT BUY THIS PRODUCT!See full review