High doses of supplemental iron (45 mg / day or more) may cause gastrointestinal side effects, such as nausea and constipation [5]. (ods.od.nih.gov)
One caveat: If you choose the oral form, the protocol involves giving your dog increasingly high doses until he develops gastrointestinal side effects or a heart rate increase. (dogcare.dailypuppy.com)
This will increase the chances of experiencing gastrointestinal side effects. (dietsinreview.com)