Same drugs, different species: How the effects of drugs vary between dogs and humans
We humans have a lot in common with our dogs. We have co-evolved, shaping each other biologically and behaviorally over thousands of years in an unusually close interspecies relationship. Dogs can read our gestures and intentions and understand much of our language. They also grow up and grow old much like we do, though at a faster pace. For as far apart as we are on the evolutionary tree, we are surprisingly alike.
These similarities include many aspects of our health. Like humans, young dogs are most likely to suffer from injuries and infections, while diseases of aging, like arthritis and cancer, afflict both humans and dogs in their later years. Not only the diseases we experience, but also the treatments for them, are often the same for us and our dogs. Clients in my veterinary practice are often surprised when I prescribe the same medications for their dogs that they or their family members take.
Same problems, same treatments (sometimes)
When my mother had a recent bout of pneumonia, she was given doxycycline, which is the same antibiotic I often use in dogs with respiratory infections. One of the most common antidepressants used in people, fluoxetine, is also frequently prescribed for behavioral problems in dogs. Like these examples, many of the drugs I use are approved and manufactured for human use and are employed “off-label” for my patients because they work in dogs too (and because there often are no drugs specifically for dogs that treat the same problems).
Sharing isn’t always caring!
Of course, we’re also different from our dogs in a lot of ways. My dog, Kalani, will never play guitar or write a blog post, and I’m pretty terrible at catching squirrels and chewing rawhide. Our differences also influence our health, from the specific ailments we suffer from to the treatments that make us better. While sometimes drugs made for humans can be useful in dogs, many human medicines can be ineffective or even harmful to our pets. Sharing is not always caring, and you should never give your dog your own medication or take theirs!
Even common, nonprescription medications can be a hazard to our dogs. For minor pain, we often take drugs like acetaminophen or non-steroidal anti-inflammatories like ibuprofen. These are safe enough in humans to be available without a doctor’s prescription, but they can be deadly to our dogs because of small, but critical, differences in how our bodies process and eliminate such medicines.
Another example are the common decongestants pseudoephedrine and phenylephrine. Humans can use these to treat a stuffy nose due to allergies or a cold. However, dogs have a much more limited ability to process and eliminate these drugs, and they can experience what amounts to an “overdose” from even small amounts. This can lead to symptoms such as anxiety and agitation or even seizures and heart failure.
Good for the goose, but maybe not the gander
Such differences in biology can also work the other way—side effects seen in humans don’t necessarily happen in dogs. A good example is a common antibiotic used in humans called ciprofloxacin. It isn’t very useful for dogs because they don’t absorb it well, so they often can’t get enough into the bloodstream to be effective. A few small changes in formulation, however, gave us the drug enrofloxacin, which is a very safe and effective antibiotic for dogs. People will definitely want to stick with cipro, though, because enrofloxacin can cause us to have very unpleasant dreams and even hallucinations!
Another good example is the drug pimobendan, which is widely used to treat heart disease in dogs. This medication has shown great benefits in terms of delaying the progression of some kinds of heart disease (e.g. mitral valve disease) and extending life for many dogs with heart failure. Pimobendan was originally developed for use in humans in the 1980s, but it is currently only approved in Japan, and it is not commonly used even there. The reason is that despite some promising benefits seen in early studies, a clinical trial in the 1990s showed that the people getting this drug were almost twice as likely to die during the study as those getting a placebo! This is exactly the opposite of the positive effect on survival seen with pimobendan in dogs, and it reminds us that drugs can have very different effects in different species.
Sometimes, drugs originally meant for use in humans turn out to have unacceptable risks. However, this can actually be an opportunity for veterinarians if these risks don’t apply to our patients. An example of this is the antibiotic chloramphenicol. Developed in the 1940s, this drug was one of the first antibiotics. Despite being effective, it is rarely used in humans nowadays because it can sometimes cause severe, irreversible anemia (failure to make red blood cells), and this is typically fatal. Fortunately, dogs don’t experience this problem with chloramphenicol. Its use is still limited because of the need to protect humans from exposure to it, but it can sometimes be a life-saving treatment for antibiotic-resistant infections in dogs.
Our dogs and ourselves
Humans and dogs are alike in many ways, and these similarities sometimes allow us both to benefit from the same medicines. However, sometimes our differences really matter. Both the benefits and the risks of drugs can differ a lot between dogs and humans. We can’t assume that any given drug will have the same positive or negative effects in dogs that it has in people.
Ideally, every drug should be tested thoroughly for both safety and effectiveness in the species we intend to use it in. This is why there are separate divisions in the Food and Drug Administration for evaluating drugs meant for humans and those meant for dogs and other veterinary species. Only such testing can give us confidence in predicting the benefits and the safety of a specific drug in humans or in dogs.
Resources
FDA Center for Veterinary Medicine database of drugs approved for animals
FDA site for reporting suspected problems with animal drugs or medical devices
The Pet Poison Helpline Poison List (includes drugs, plants, and more)
Forbes article on dangers of humans and pets sharing drugs