Senior Dog With Heart Problem and Effects of Iron Tablets

[maggiemcgill]

My Jack Russell (elderly) has recently been diagnosed with an enlarged heart; this is pushing against his wind pipe making him cough rather deeply. Other than the tablet for the condition the Vet advised an iron supplement; since then his stools tend to be black, and this evening after his feed he vomited. I researched the net but could only find side effects for humans, which included dark stools and vomiting, weak/fast heart rate was also included. Would the heart rate side-effect work the same way for a dog, if so is it wise with a dog that has a heart condition to be taking these? Any alternative suggestions or help would be gratefully appreciated, Thank You.
p.s. I must add that despite his age he is an extremely sprightly fellow & when on walks can put us to shame