Which Is Better: Azathioprine or Mycophenolate?

Which immunosuppressive drug is better to treat veterinary patients: azathioprine or mycophenolate? Andrew Mackin, BVMS, MVS, DVSC, DACVIM, professor and department head at Mississippi State University College of Veterinary Medicine, says it has a lot to do with your previous experiences.

“My presentation yesterday was azathioprine versus mycophenolate: which is better? And I actually admitted to the audience that was just to get them in, because there really is no better drug. One is not better than the other, they're both reasonable choices. And the main theme of what I talked about is how, in many instances, they're interchangeable—you can use 1 or the other.

I think that people are biased by the last failure that they had or the last side-effect that they had. It'll turn them off 1 drug and they want to find that miracle drug that does better. In reality, they both have a reasonable chance of success and they both have a reasonable chance of causing side effects and both can be used in similar circumstances. If you're worried about something that distresses the owner, which is bad diarrhea, mycophenolate is probably not your first choice. However, if you're worried about creating something that worries you, which is a significant elevation and liver enzymes in some dogs, don't use azathioprine because that's the side effect that will scare you.

And then if a dog has a reaction to one of the drugs switch to the other, whichever one you like the other one is a reasonable alternative if they don't tolerate the first drug. The only absolutes that I say, is don't use the 2 drugs together. Veterinarians tend to like to stack drugs together and that's not a bad thing, it's commonly done in human medicine, but those 2 drugs are so similar that if you use them together, it's like a double-barrel shotgun—it has too many side effects. Never use them together.”