These are amazing.  We battled fleas and even with treating the yard and house each month as well as the animal we still had fleas.  Tried the collar on one dog and no fleas.  Now all the dogs and indoor cats have them on and after a full year of use we have not had to treat the house or yard after about the third month.  Well worth the investment.  No issues with senior dogs (ages 2 years to 17 years) and cats (20 and 17 years).