
You won't know unless you try! They work for some dogs and do nothing for others.
Some tips. It can be a good idea to try a tight-fitting child's t-shirt first, it's a hell of a lot cheaper and you'll get an idea if it's going to work from that (or indeed that might be enough!). Another option is a body wrap - google ttouch body wrap. Works on the same principle and it can be a bit more customised. Also, the velcro on thundershirts is immense and can cause problems by itself - we tried one on Phoebe in her last home, it worked really well the first time but once it was taken off, the noise of the velcro upset her and now she won't tolerate anything at all, even a thin body wrap. And it's not just noise phobics like her - same thing happened with another client's dog who has no noise issues whatsoever. I'm not the only person who's seen this happen. But a body wrap just knots at the ends, so no velcro. Damn sight cheaper too!
If you do buy a thundershirt, get it from the company direct - they offer a full refund if it doesn't work, or at least they did. Do check that first. At £40ish a time it's a huge layout if it doesn't then work, and a lot of places won't refund you once it's been used.