Sometimes, the owner is slow to come clean about the cause of the dog’s malaise.

“I tell them, ‘Look, I’m not the police, I really don’t care, it’s just going to help me diagnose your dog a lot quicker,’” Dr. Levitzke said. He explained that a confession could also save money on tests for more serious conditions like kidney failure and liver disease that produce similar symptoms. Often, Dr. Levitzke said, owners will say the animal got into their roommate’s stash.

Harry the poodle’s vet, Richard Goldstein, the chief medical officer at Animal Medical Center, said that a trip to the veterinary emergency room can lead to some tense family discussions.

“Sometimes the parents find out that the kid had some, and an interesting family dynamic develops,” Dr. Goldstein said. “I’ve had cases where the parents asked me to talk to the kid to explain the repercussions of the kid’s carelessness.”

For Ms. Gotbaum’s part, she swears she last smoked marijuana circa 1976 (she could not stop laughing and hated it).

“I would tell you — I don’t care, makes no difference to me anymore,” said Ms. Gotbaum, who is 78 years old and has not held public office since 2009. Her live-in boyfriend, Peter Lewis, said he never tried it, as did their housekeeper, Julia Alvarez, who often takes Harry for his walks.

The household’s working theory: Harry scarfed something off the ground the night before on a walk to Central Park. “In the park, I’m always smelling marijuana,” Ms. Alvarez said.