Glucosamine and chondroitin are dietary supplements used (ostensibly) to treat and prevent arthritis in dogs, cats, and people. Glucosamine is one of the main components in cartilage. Cartilage is a main component of joints. Cartilage wears down in arthritic joints. In theory, glucosamine may help support cartilage, thereby slowing the progression of arthritis. Some plumpers for the supplement also claim that it might have anti-inflammatory effects.

The theory behind oral glucosamine never quite worked for me. Orally ingested glucosamine must run the gauntlet of digestive enzymes once it’s consumed. I never quite bought the notion that it could make it to joints unscathed and still effective.

However, I (and millions of pet owners, veterinarians, and human arthritis sufferers) really wanted glucosamine to work. It was a natural product. It was less toxic than NSAIDs (although a recent letter to the editor of the Journal of the Veterinary Medical Association linked the product to liver failure when dogs overdosed on it).

Most important, hundreds of clients offered feedback that the product worked well in their pets.

That feedback, it turns out, is more likely a testament to the power of the placebo effect than to the beneficial effects of oral glucosamine. And yes, animals can experience the placebo effect — when an owner thinks, wrongly, that a product has made his pet feel better.

A very large meta-analysis (a study that combines the results of a number of other studies) recently was published on the use of oral glucosamine and chondroitin in humans. The conclusion: glucosamine is snake oil. Here are the authors’ conclusions.

Compared with placebo, glucosamine, chondroitin, and their combination do not reduce joint pain or have an impact on narrowing of joint space (which is a measure of the progression of arthritis). Health authorities and health insurers should not cover the costs of these preparations, and new prescriptions to patients who have not received treatment should be discouraged.

They don’t mince words. I can think of no reason why the results would not apply in cats and dogs.

Like I said, I wish oral glucosamine worked. I want it to work. But I want many things that I cannot have. Effective arthritis treatment with oral glucosamine evidently is one of them.