If you are in an area that's truly infested with ticks, the Preventic Tick Collar is the answer. This is no light-duty collar, and utilizes amitraz as an active ingredient which is the most effective tick preventative available. This collar will cause existing ticks to detach and die within 24 hours and keep any new critters from getting close to your dog! We use this collar and highly recommend it.
Medium - Dog neck size up to 18" 1 Collar each
Large - Dog neck size 18" to 25" 1 Collar each