My cat vomited pink liquid and is lethargic after Onsior. What to do?

Sorry to hear about Mr. President. He may have an adverse effect from the Onsior, but is difficult to tell without knowing all of the previous history. I would expect a cat that is hurt to be less active and eat less, but if this was not a problem prior to the vet visit, then I would seriously question the treatment given.

Onsior can cause adverse effects to the stomach and small intestines; which can lead to a lack of appetite, vomiting and gastrointestinal ulcers. I am concerned that the vomit was pink, because this can possibly be due to bleeding. However, bleeding in the stomach is usually seen as black, coffee-ground-like specs, so this is not typical of an ulcer.

It's best to take a picture of the vomit (if you still can), so you can show your vet. I would recommend you have him rechecked by your vet within the next day to ensure everything is OK. They can help determine if anything different needs to be done.

In the meantime, make sure to give food and water as normal and don't attempt to give any treatments without a veterinarian's direction.

I hope that helps and Mr. President gets better soon.