When she was 3, this Great Pyrenees-Bernese mountain dog mix ate something she shouldn’t have. Photo by Stacy Zarin-Goldberg

For David Nellis, taking his Great Pyrenees-Bernese mountain dog mix, Lucy, for a walk is like getting pulled along in a sled dog race. She walks him. At 175 pounds, Lucy is more like a polar bear than a pet. Sometimes her size can be a hassle, Nellis says, but at least once it’s saved her life.

One night in 2013, Nellis and his wife, Nycci, went out to eat, leaving Lucy and their other two dogs—Prudence, a smaller Great Pyrenees-Bernese mix, and Daisy, a pit bull mix—at home. Earlier that day, the Kensington couple had picked up a three-month supply of phenobarbital, an epilepsy medication, for Daisy and left the pill bottle in a paper bag on a high shelf in the kitchen.

After dinner, they returned to find the bag on the floor, in shreds, and the bottle broken apart. Three-year-old Lucy was staggering around with bits of pills and plastic in her mouth. As a puppy, she chewed on phones and remote controls—“in her younger days, she was a holy terror,” Nellis says—but she’d never done anything as serious as this. The dog had eaten between a quarter and a half of the 180 pills in the bottle.

“I was sort of counting on the fact that she was big,” Nellis says. “If she was a skinny dog, she would have died—she probably would have died within an hour.”

After trying unsuccessfully to induce vomiting, Nellis drove Lucy to the Metropolitan Emergency Animal Clinic in Rockville. Then the dog collapsed. “I got her out of the truck, and as we got into the vet hospital she just went down,” he says. “She went into a coma. Thank God [it happened then], because I wouldn’t have been able to carry her.”

Dr. Sara Thompson, who treated Lucy, could tell right away that the dog was suffering from a substantial overdose. Since phenobarbital is a respiratory depressant, Thompson knew that Lucy might stop breathing, and that even if they put her on a ventilator, it could be too late. “It was one of those [cases] that stands out in your mind,” she says. Dr. Tina Wismer, medical director of the ASPCA Animal Poison Control Center, which Thompson contacted about Lucy, says the organization gets about 170 phenobarbital overdoses a year. Few are deadly, but it’s rare that any animal ingests a dose as large as Lucy did.

Veterinary staff hooked Lucy to IVs to keep her hydrated, but there wasn’t much else they could do. “They told us she probably wouldn’t make it through the night,” Nellis says. “She’s just so big and tough—I didn’t buy it.”

Nellis, 67, lay on the floor with Lucy the entire night. Her eyes were glazed over and her body was limp, but once in a while it seemed like she was responding to him. “Every time I said her name and said, ‘Come on—let’s go outside!’ her heart rate would go up and her breathing would go up,” he says. “And then it would go down again.”

After two days in a coma—veterinary staff eventually brought Nellis a cot to sleep on—Lucy slowly started to wake up and tried to move. Soon she was back to her old self, sitting with the Nellises every night while they watch TV, and she remains healthy three years later. “She’s been like this ever since,” Nellis says as Lucy lets out a deep, bellowing bark. (It’s mostly for show—she’s actually a bit timid and is afraid of small animals.) “I’ve had lots of dogs, and Lucy is the most amazing animal I’ve ever had.”

The Nellises keep medications tucked away now, out of reach of Lucy, Prudence and Daisy, as well as their newest dog, Maisie, who’s been known to eat shoes.