If you're not deficient in iron, you might try switching to a brand of vitamins that don't contain iron. (momtricks.com)
The top five brands we mentioned above come highly recommended by numerous dog owners who have seen significant health improvements after switching to these brands. (ipupster.com)
Your vet may also suggest switching to a brand with a different protein source or grain component to see if the reason for the upset stomach is allergy - related. (animalplanet.com)