Pet placebo?

[IvanV]

My wife obtains these non-prescription capsules called Zylkene, made by a firm called Vetoquinol. They are advertised as reducing anxiety in companion animals. The cat is given them for a few days before we take her on a journey in the car. Although also labelled in English, they are mainly sold in Czech and Slovakia, which is where she buys them. The same product is sold for cat and dog, the only difference being the quantity in the capsules.

The ingredients are clearly labelled: maltodextrin, casein and magnesium stearate.

This looks like a placebo to me. But can placebos work on non-human animals? To get the cat to eat them, we have to open the capsule and scatter the powder on her food. So is the cat even aware she is taking them? Perhaps they taste of something. But it doesn't affect her eagerness to eat her cat food. Cat food has special smell and flavour substances added to it, that makes it very attractive to cats, but disgusting to humans. Can you taste a little bit of maltodextrin, etc, through that?

Perhaps the target of the placebo is actually the owner. We might propose a mechanism similar to "audiophile" products. With those, you do something that you have been told will improve the sound of your hifi, though frequency sampling shows no objective difference. But by drawing your attention to it, and sequential testing, you magically perceive that it has improved with the treatment. Here we hope to perceive that the pet is calmer than she otherwise might be on a car trip. She usually calms down eventually. After an hour or two in the car.

[Grumble]

Placebos don’t really work on humans, never mind non-humans. The point of them is to find out what happens with no treatment.

[Fishnut]

My understanding is that the placebo impacts the subjective interpretation of the owner with regards their pets health. They think their pet is doing better whether or not there's an objective change.

[Sciolus]

It's also possible that the placebo will affect how the person behaves with/around the animal, which it turn affects its behaviour. In the OP, it might make the owner more patient which might have a calming effect. ("Possible" and "might" are doing a lot of work there.)

[IvanV]

Placebos don’t really work on humans, never mind non-humans.

Placebo effects (Wikipedia) It depends what you mean by "really work". Meta-analyses conclude that there is no good evidence that they can promote the objective healing of physical disease or injury. But there is good evidence that they can have an effect on pain, and potentially other things where subject perception is relevant, such as depression.

And then, "Research consistently shows that patients who maintain a positive mindset often experience better health outcomes, including faster recovery..." (source). Which kind of suggests that there ought to be some kind of a secondary route from placeboes to physical healing, if the placebo can help with a positive mindset.

There are these curious findings.

Placeboes have greater effect the more expensive and/or elaborate they are.
Placeboes can still work even when the patient knows they are a placebo.

Motivated by the later, during the period of my life when I had episodes of depression, I used to take St John's wort, and referred to it as "my daily placebo". I eventually had to stop when I realised that it was contra-indicated for one of the regular meds I was subsequently prescribed for a chronic condition.

The point of them is to find out what happens with no treatment.

You've got that a bit mixed up. Because a placebo can have an effect, the point of using placebos in placebo-controlled trials is to find out if the test treatment can have an effect beyond a placebo. As noted in the wiki article cited above, on average about 82% of the effect of depression meds is placebo effect, which is not zero.