Within one week, we have had two known severe reactions to Sentry Flea and Tick medication. This is the medication that is applied once a month to the back of your pet's neck to kill fleas and ticks. My daughter and her husband applied Sentry to their Golden Retriever about one week ago. Within two days, the dog began scratching incessantly, shaking, and then developing a quarter size open sore approximately 3 inches from the application site. After an expensive trip to the Vet, they were told NOT TO USE THIS PRODUCT as it can be lethal to pets. Their dog was prescribed and antibiotic and steroid to diminish the symptoms. Before I knew that they had taken their dog to the Vet, we administered Sentry (bought and applied before knowing about my daughter's dogs diagnosis), to our lab. Within 12 hours, our dog was shaking his head and hears, shaking, and scratching non-stop. He began acting ill and not eating or drinking. Within 24 hours, he ALSO developed a sore approximately 3 inches from the application site. When I contacted my daughter, she informed me of their very similar experience with Sentry. I then googled "Sentry Flea and Tick" and found a huge number of complaints about the product from pet owners who had experienced many of the same symptoms...convulsions, tremors, lethargy, frothing, scratching, and neurological issues. Many of these people even lost their dog due to the medication's massive toxic side-effects. DO NOT USE THIS PRODUCT! See your vet and only purchase from an experienced medical pet care provider. We have learned an important lesson and hope to get the word out there to those who love their pets like we do.