Selective serotonin reuptake inhibitors (SSRIs) like fluoxetine are now commonly prescribed for dogs with severe separation anxiety or compulsive tail-chasing. For cats with feline hyperesthesia syndrome (a neurological condition causing rippling skin and self-mutilation), gabapentin or phenobarbital can restore quality of life.

When we treat behavior as medicine and medicine as behavior, we do more than stop biting or barking. We relieve suffering. And that, after all, is the oldest promise of veterinary science. If your pet shows a sudden behavior change, consult your veterinarian to rule out underlying medical causes before seeking a trainer or behaviorist.

Consider the dog who suddenly starts drinking from puddles. An owner might see a bad habit. A savvy veterinarian sees a potential case of Cushing’s disease or diabetes. Consider the elderly cat who begins yowling at 3 AM. This is rarely "being mean"—it is often the first sign of hypertension or cognitive dysfunction syndrome.

Conversely, behavioral problems cause physical disease. Chronic anxiety in a dog leads to elevated cortisol, which suppresses the immune system. A stressed horse that crib-bites wears down its teeth and risks colic. A parrot that plucks its feathers opens the door to bacterial infections.

A dog with a thyroid imbalance may seem hyperactive and untrainable. A cat with dental disease may suddenly start eliminating outside the litter box, not out of spite, but because the pain of clenching to urinate is unbearable.

However, veterinarians stress that drugs are not a solution alone. They are a tool to lower an animal’s arousal enough that learning can happen. "Medication without behavior modification is a missed opportunity," says Dr. Vasquez. "But behavior modification without medication, when the animal is panicking 24/7, is cruelty." For pet owners, this new science offers hope and responsibility. If your pet’s behavior changes suddenly—especially after age seven—do not reach first for a trainer or a shock collar. Reach for your veterinarian.