Prescription drugs indicated for use in veterinary medicine can be dangerous for humans. Conversely, medications approved for human use can pose a serious hazard for animals. And at the same time, some therapeutics can be prescribed to humans and animals and may work for both, so long as this is done with the appropriate dosage and formulation.
Ever wondered about the fact that your cat may be taking the same medication as you are? For example, cats can be administered transdermal doses of mirtazapine (brand name is Remeron or Mirataz) as an appetite stimulant. Mirtazapine is approved by the Food and Drug Administration for the treatment of depression in humans. Appetite stimulation and weight gain are common side effects, which is why it can be beneficial for patients who are experiencing weight loss and decreased appetite. Similar benefits may be achieved for animals, too.
Veterinarians can legally prescribe an approved human drug such as mirtazapine in animals under certain circumstances. This is called an extra-label use, as the American Veterinary Medical Association explains. It entails prescribing of an approved drug in a manner that deviates from the drug’s approved labeling, yet meets the conditions set forth by the Animal Medicinal Drug Use Clarification Act of 1994 and FDA regulations. Here, deviations from FDA-approved labeling include use in another species, for another indication, at a different dose or frequency or route of administration.
The FDA is the legal authority in the U.S. to approve and regulate drugs for both humans and animals. A drug intended for use in animals is called a new animal drug. The FDA has a division named the Center for Veterinary Medicine which approves and regulates new animal drugs. And because every animal species responds differently to pharmaceuticals due to differences in physiology and metabolism, the FDA determines if a drug is safe and effective for a specific use in a particular animal species.
According to the National Community Pharmacists Association, the following four drugs that were developed and approved for humans are commonly administered to certain animals under the extra-label rubric: Diphenhydramine to help treat allergies, allergic reactions and motion sickness; hydrocortisone for raw, itchy or irritated skin; famotidine as a stomach acid reducer; and dimenhydrinate for motion sickness, though a better choice might be an FDA-approved animal treatment such as Cerenia (maropitant citrate).
But you don’t want to ever play veterinarian with the human drugs in your medicine cabinet. Consult your vet before using any of the above drugs for your pets.
Nor should you ingest drugs intended for animal use just because you may recognize a familiar active ingredient. In other words, people shouldn’t use products marketed for veterinary use that have not been evaluated by the FDA for human safety or that are otherwise not suitable for human consumption. Those products may have adverse effects, including serious illness and death, when taken by people. Either the veterinary drug itself poses a considerable risk to humans, or the dosage or formulation.
Take xylazine, for instance. It is a non-opioid veterinary tranquilizer which is approved for animal but not human use. Not only is the animal sedative dangerous when taken on its own by humans, fentanyl mixed with xylazine is known on the street as “tranq” and is fueling an alarming rise in overdoses and deaths.
Another example is the dissociative, hypnotic drug ketamine, which can be legally prescribed to humans for certain mental disorders. But ketamine has also become a street drug for illicit recreational use. There are veterinary and human formulations of this anesthetic. The veterinary formulations are ten times stronger, which makes it potentially deadly if ingested by humans.
The reverse applies, too, that is, when animals use products intended for human use only, they can experience adverse effects such as serious illness and death. Some drugs are highly toxic and potentially lethal for cats, for example, bismuth subsalicylate and acetaminophen. The FDA lists drugs that are prohibited from extra-label use in animals.
In other instances, a drug such as fluoxetine—the active ingredient in the commonly used antidepressant Prozac—may have approved on-label uses in both humans and animals. For “lonely dogs with separation anxiety,” Eli Lilly marketed Reconcile in 2007, which contains fluoxetine and is specifically formulated for animals. Dogs are often prescribed fluoxetine. This isn’t a new phenomenon. Fifteen years ago the New York Times published an article on the rise in the practice of prescribing medications designed for humans to animals.
And then there’s the special case of the broad spectrum antiparasitic ivermectin which has long had approved uses in humans and animals. However, animal ivermectin products are very different from those approved for humans. In veterinary medicine, it is indicated to prevent and treat heartworm and intestinal worms. You may have seen ads on television for Heartgard, which prevents heartworm disease in dogs, and treats and controls intestinal worms in animals. One of two active ingredients in Heartgard is ivermectin.
In humans, a topical ointment containing ivermectin can be prescribed to treat issues involving lice and rosacea, while a tablet is used for parasites, including intestinal strongyloidiasis and onchocerciasis (river blindness) and lymphatic filariasis.
During the Covid-19 pandemic a controversy erupted around the use of ivermectin in humans to treat or prevent Covid-19. The FDA in December 2021 warned Americans not to use ivermectin for this purpose. The FDA stated that there wasn’t evidence to support ivermectin’s use against Covid-19.
The agency went further by reminding us “never to use medications intended for animals on yourself or other people.” While the latter statement is apt, technically it doesn’t apply to ivermectin as the product isn’t only for animals. Moreover, the statutory authority of the FDA does not extend to the issuance of medical advice or recommending against off-label uses of medicines.
While the FDA does not approve of ivermectin as a treatment for Covid-19, doctors can still prescribe it if they insist, however ineffective it appears to be for patients, as has been chronicled repeatedly in separate peer-reviewed studies during the past several years.
The FDA threw down the gauntlet when it posted a tweet in 2021 opposing the use of ivermectin: “You are not a horse. You are not a cow. Seriously y’all. Stop it.” This drew the ire of some physicians who believed the FDA had overstepped its boundaries. In a lawsuit, the FDA has since agreed to delete and never republish the infamous tweet and other similar posts on social media, according to Newsweek.
Broadly, we can say that drugs that have approved uses in both humans and animals can be prescribed, so long as this is done with the appropriate dosage and formulation. Further, human medications can be used for pets and other animals in a limited number of instances—under the extra-label regulation—but only if recommended by your veterinarian and dosed and formulated appropriately. Finally, medicines intended for use in animals only shouldn’t be taken by humans.
