A Texas death row inmate is challenging the substitution of pentobarbital for the more traditional sodium thiopental as the initial, anesthetizing drug in his three-drug execution cocktail. The state’s supply of sodium thiopental expired in March, and there is a worldwide shortage of the chemical agent. Pentobarbital, the proposed replacement, is commonly used in animal euthanasia. Do doctors and veterinarians use a lot of the same medications?

Absolutely. Dogs, cats, horses, and sheep regularly take the same medicines as wounded bipedals. Many, and perhaps most, antibiotics are approved for use in humans and animals. Versions of some of our anti-anxiety medications and painkillers are approved for other species as well. But just because the active ingredient is the same in human and animal formulations doesn’t mean the pills are identical—there’s often a difference in dosage, and the inactive ingredients may differ (in kind and amount) as well. So the state of Texas won’t buy its pentobarbital from the animal hospital, even though there is a super-concentrated veterinary dose specially formulated for euthanasia.

Many animal drugs start off as human medications, because there is far more money invested in human drug research. In such cases, manufacturers sometimes reformulate their drugs for sale to animal owners. For example, if the dose for dogs is less than for humans, they may have to add more binding agent to bulk the pill up to a manageable size. (The active ingredient constitutes a minuscule fraction of the overall volume of any pills.) The company might also put more or less of a chemical that aids in absorption.

In other cases, large companies working in both the medical and veterinary fields develop the two formulations side by side. The approval processes for human and animal drugs are entirely separate, although similar. Just like ordinary medicines, veterinary drugs have to clear randomized, blinded, placebo-controlled trials. Animal trials, however, often contain about one-tenth as many patients. (On occasion, pharmaceutical companies don’t bother with the animal approval process, because there isn’t much of a market for veterinary cancer drugs, for example. Vets have to prescribe those medicines off-label.)

Pills destined for animal use may also contain specific additives. Humans take the anti-inflammatory drug meloxicam in pill form, while dogs consume it as a liquid slathered over their dinners. To prevent Fido from turning his nose up at the medicated feast, the company includes a honey flavorant that is not approved as part of the human formulation.

So how dangerous is it for a human to take animal medicine? The official answer is that you shouldn’t take animal drugs. Even if you could accurately diagnose yourself, figure out the appropriate dose, and adroitly cut the horse pills down to the right size, it’s hard to tell whether the amount and type of inactive ingredients in any particular drug are safe for people.

That said, while neither the government, the pharmaceuticals industry, nor the Explainer himself recommends taking veterinary medications, the truth is that, in some instances, if you get the dose right you’ll be fine, since a good many animal pills contain exactly the same ingredients as their human analogues (just in different proportions). Reckless readers may be interested to know that many agricultural supply stores contain a refrigerator case stocked with antibiotics and vaccines approved for use in livestock. These medicines, along with fish antibiotics, are available without a prescription, much to the chagrin of doctors, veterinarians, and public health advocates.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Terry Clark of Nexcyon, Dennis Geiser of the University of Tennessee, Mark Papich of North Carolina State University, Ron Phillips of the Animal Health Initiative, and James Schafer of Schafer Veterinary Consultants.