I took the Ferris Sulfate, my PCP told me to get the standard OTC and I had enteritis (that's a really nice name for diarrhea) for 3 months and ended up in the ER because of it. Terrible GI upset from that stuff, not too good for me on top of the anemia.
My doctor then told me to try the liquid but it was a low dose, that's why it is well tolerated I think. My GI doc put me on Poly Iron 150mgs 2X=300 mgs daily for the last year minus the times I was ill and couldn't take anything at all. It is available OTC but you have to ask for it at the pharmacy. It also has Vitamin C already added so you don't have to worry about that. My year script just ran out so I had to ask at the pharmacy and they tried to sell it to me as Poly Iron Forte form, but I realized it on the spot. The difference was is that it also has B-12 added if you need that but I didn't. So they had to special order it which only took one day. It is well tolerated even in high dosage as I was taking 300mgs elemental iron daily. I now take 150 daily until menopause. At least that's what my Hemotologist prescribed. I'm not sure how I feel about continuing iron supplements in that way. I think your doc is right about the sulfate kind.
Everyone reacts differently and you have to try them out and see what works well for you. I think that the script iron are the best because of the fact that they have C already in it and they are a higher dose verses OTC. But the low dose iron don't do the job if you are really anemic like I was and a low ferritin, mine was a 2. When I was anemic it took me 2 1/2 months to raise my levels in response to the supplements. That was when I was able to take the correct dose as prescribed. FLFLOWERGIRL