Our three labs have them on and so far so good! One has had his on for about a month and the other two only days but it is beginning to be flea and tick season and we want them to be protected early. The one who has had his for a month does not have any flea dirt, fleas or ticks and we check him all the time.  We have also looked for any skin reactions and there are none on any of them. We are very happy with this product and price wise, it's a good deal since it will last up to 8 months