How Thundershirt Works; Thundershirt uses gentle hugging to calm your dog or cat. With its patented design, Thundershirt’s gentle, constant pressure has a dramatic calming effect for most dogs and cats if they are anxious, fearful or overexcited. Based on surveys completed by over two thousand customers, over 80% of dogs and cats show significant improvement in symptoms when using Thundershirt. Thundershirt is already helping hundreds of thousands of dogs and cats around the world and is recommended by thousands of veterinarians and trainers. How does Thundershirt work? Experts believe that pressure has a calming effect on the nervous system, possibly by releasing a calming hormone like endorphins. Using pressure to relieve anxiety in people and animals has been a common practice for years. Thundershirt's adjustable fasteners allow for a snug fit to many different body shapes.