Awww Emma, its not fun is it? We went through the same thing with DS a week or so ago. I switched him to Step 2 and within days he was screaming the house down he was in so much pain. He was already having constipation issues on the Step 1 anyway but we had it under control by adding lactulose syrup to his morning bottle daily. Its a syrup that helps retain water in the bowel and helps to keep their poo runny. You can buy it from the chemist. Our GP recommended it.
So after that experience and doing my own research, we'll be keeping DS on Step 1 until 12 months then going to cows milk. If it ain't broke as they say. According to the tins the main difference I noted was in the levels of minerals and as far as I'm concerned if he's getting a good diet of solids, that will be taken care of. Plus as Curly said the Step 2 are a casien based formula which is harder for them to process again.
If I were you I would ignore what the chemist said (particularly if they couldn't give you a good reason) and stick to the original Step 1 you've been using to try and help get the haemmoroid under control. If your GP is not much help do you have a MCHN near you that might be of more assistance? Most of the major women's & childrens hossy's in each state usually have a phone line you can ring for anything related to babies too?
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