My new puppy has loose stools after diet change. How to firm poop?
This does sound like a colitis, which can be due to an abrupt diet change, but it's also possibly due to something else like and infection or toxin ingestion.
I think it would be worthwhile to switch to the Pedigree and see if that helps. If that helps, but you want to switch him to the Iams in the future, then you can try doing it again slowly. Below is a link to a good article that explains how to do this.
https://www.petcoach.co/article/how-to-transition-your-dog-or-cat-to-a-new-food/
If the pedigree doesn't work, then you can try feeding bland food (boneless chicken and rice is good). Feed small portions (1/2 cup or less) three times a day. You can also add probiotics to the food (can get this at the pet store), which has demonstrated some efficacy for improving diarrhea. Ensure fresh, clean water is always available.
Diarrhea is a common problem with puppies and is usually self-limiting (will go away on its own). It may not be serious right now, but it's always possible that it could be the start of something serious. If Gizmo has any of the following, then he should see a vet:
1) Blood in diarrhea
2) Very watery diarrhea
3) Starts acting very depressed
4) Stops eating or drinking for longer than 24 hours.
5) Diarrhea lasts longer than three days.
I hope this helps and Gizmo gets better soon.
Updated on September 23rd, 2025