I am not concerned that the two medications were given at the same time.
The only thing I am concerned about is the use of a laxative for a cat. This can sometimes predispose to bad abdominal cramping and bad diarrhea.
Generally, to help avoid constipation in cats, vets try to encourage greater water intake and increased fiber intake. Examples include adding canned pumpkin in the diet or using stool softening medication like lactulose.
So, just as a side note, I would ask your vet about these options if you run into any side-effects or problems with the miralax.
But, no, I am not worried they were given together.
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