Veterinarians routinely use drugs “off label.” When a company wants Food and Drug Administration (FDA) approval for a new medication, it has to prove that the product is both safe and efficacious. The approval process is long, complicated, and expensive. To simplify matters the company typically picks the most prevalent (lucrative) condition that the drug could be used to treat and runs with it.

Once the medication is on the market, veterinarians start thinking outside the box. With knowledge of the drug’s mechanism of action, the physiology of veterinary patients, and how related compounds are used, doctors will try it for other conditions. This isn’t as risky as it might sound (and it is perfectly legal) since the initial FDA application and subsequent scientific studies and/or clinical use have demonstrated that the drug is safe (or mostly so … more on this later). The question is, “Will it work for a condition other than the one(s) listed on the label?”

Here’s a real world example. Maropitant (Cerenia) is a relatively new veterinary drug labeled for the treatment of nausea and vomiting. Maropitant belongs to a class of drugs called neurokinin (NK-1) antagonists. There are many different types of neurokinins, but the important one with regards to maropitant goes by the enigmatic name, “substance P.” Substance P is a neurotransmitter involved in vomiting. By blocking it, maropitant can stop vomiting. But substance P is also found elsewhere in the body, especially in mast cells that play a big role in allergic reactions and inflammation.

A few enterprising veterinarians have started experimenting with using maropitant for other substance P-related conditions including allergic skin disease, sinusitis, joint disease, feline interstitial cystitis, coughing, diarrhea, and more. Initial clinical findings indicate that the drug may be beneficial, particularly in combination with rather than instead of more traditional therapies.

But here comes the potential downside of off-label use. Substance P is also involved in the functioning of the central nervous system (CNS). Using maropitant every day eventually depletes the reserves of dopamine in the CNS and can lead to tremors (think Parkinson’s disease). Since the longest approved duration of use on the product’s label is five days, this wasn’t a problem until doctors began using the drug off label. Veterinarians have determined that giving the medication on a schedule of five days on–two days off, or every other day, prevents this side effect.

I don’t like my patients to be guinea pigs. Clinical experience with the off label use of maropitant is in its infancy so I’m waiting for more information to be available before I try it for anything other than nausea and vomiting. I’ll be watching the patients I do use it on closely though to see if their allergies or other concurrent diseases improve.

Dr. Jennifer Coates

Image: shoe the Linux librarian / Flickr