After Eugene Ovsishcher returned from a nine-month combat tour in Afghanistan, he experienced what his doctors called symptoms of post-traumatic stress: nightmares, flashbacks and a pervasive anxiety. A psychiatrist advised him to get a dog, and last August he did — a shaggy, mocha Shih Tzu puppy that Mr. Ovsishcher named Mickey because he crawled like a mouse.

The dog proved to be the right medicine, Mr. Ovsishcher said: Mickey woke him from nightmares by sensing something was wrong and barking, settled him down when he was alone and anxious, and even checked up on him “like a registered nurse” when he had a fever.

“Take a look at his face,” Mr. Ovsishcher said, comparing Mickey to Chewbacca, the hairy character in the “Star Wars” series. “You can’t stay anxious or angry or whatever. You look at that face and you start laughing.”

But now Mr. Ovsishcher is facing eviction from his three-bedroom co-op at Trump Village in Coney Island, Brooklyn, because the housing complex has a no-dogs policy. He is wrestling with a kind of Sophie’s Choice: his home or his dog.