Yes, it will stop a dog. I don't think it hurts them too badly though...more just stuns them.

I had the occasion to use Halt. I live way out in the country, and some of the stupid rednecks down here let their dogs run around the "back 40" and chase anything that comes by on the road. Well, one day when commuting home from work, I had a dog come out of no-where and start chasing me. He was--what I thought at the time--trying to nip at my feet. I tried to out pedal the pooch, but it wasn't working. I remembered a defensive move that I read in a book one time. Stop, dismount, and put your bike between you and the dog as a barrier. I did just that, and the dog lunged at me...(now at this point, I was even past the owner's assumed property line...up to the next house). I carried a can of Halt on my handlebar, so I reached for it and sprayed the dog right in the face...I think some of it even went in his mouth. He didn't yipe, but immediately stopped the agressive behavior and backed up. I got on my bike and pedaled like h*ll away from him.

One more point, I sense some of the posts being sympathetic to the dogs that are sprayed by this stuff. What's this about? ! ? If a dog comes lunging after you, I think you have ever right to take whatever action you can do defend yourself. Heck, if I could of smacked the dog with my heavy commuter bike, I probably would have. I don't like to harm dogs anymore than the next person, but you just never know how a running/nipping dog is going to act. But I blame dogs like this on the owners.