There are plenty of things we don’t quite understand about the placebo effect — for instance, the fact that it’s getting stronger in the U.S. — but one of the strangest may be this: It’s not just humans that respond to it. As Martha Henriques recently reported for BBC Earth, some scientists believe that placebos can work their mysterious magic on pets, too.
The more you think about it, the stranger it seems: The placebo effect is built on the patient’s belief that they’re receiving something to make them well. Animals don’t have the conceptual understanding to make that belief possible. A dog, for instance, has no idea that the peanut-butter-covered oval it’s gobbling is a pill, and that pills are consumed to heal. And yet trials for veterinary drugs often follow the same format as human trials, with one group receiving the real therapy and another group getting a placebo — and in some cases, the placebo group ends up showing improvement. In one 2010 study in the Journal of Veterinary Internal Medicine, for instance, researchers testing a treatment for canine epilepsy discovered that dogs in the placebo group were having fewer seizures than when they’d started the trial.
But that doesn’t necessarily mean the dogs were reacting to the placebo itself: It’s possible that many of the dogs would have simply gotten better over time whether or not they were taking placebo pills. “Epilepsy is a waxing and waning disease with a natural course,” veterinary neurologist Karen Muñana, the study’s lead author, told Henriques. “What happens in epilepsy is that owners will seek care when their dog’s disease — when their seizures in this case — are at the worst.” As they naturally recover while taking placebos, then, the upswing can become a case of misplaced cause-and-effect.
It’s also possible, Henriques added, that placebos change the behavior of pet owners more than anything in the animals’ physiology: “The owner who anxiously monitors their pet throughout its time in the trial might in fact be helping the pet get better” by paying more attention to its symptoms and needs and responding accordingly, she wrote.
Still, there’s other evidence for a more direct effect: In one 1982 study, she noted, researchers gave mice with immune disorders a cocktail of drugs and sugared water; eventually, the mice began receiving only the sugared water, and yet their health didn’t drop back down. And a 1999 review paper on the placebo effect found several similar lab-animal studies with similar takeaways: Somehow, the placebo effect really is working in other species, even if we don’t understand how. Chalk it up to one more thing that humans and animals have in common.