Harkers 3 in 1
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I understand. I think it is a great idea to have things on hand!
IMO it is better to have medication on hand to treat each specific symptom/illness as it arises instead of using one size fits all type of thing. This will give you better control over dosing and emliminating treating with something that may not be needed at the moment. I understand that purchasing each is going to be more $$ and I'm not saying that the 3 in 1 is a bad product, if that's what you choose it is better than not having anything at all on hand. Again this is all just my opinion. Hope this helps.
I have pulled some info from here on BYC and Ebay UK that may help. It is your first aid kit, so this is just some general things to consider.
First of all @casportpony is great with info on dosages and medications, you can always look up threads and articles by her to help determine what meds can be used, etc. Here is a link for that:
For COCCI the standard med is Corid or Amprolium, here is dosing for that and what it looks like on ebayuk:
It looks like for "canker" metronidazole would be best treatment for that so here is info for that as well:
http://www.backyardchickens.com/t/773308/canker-treatment POST #4
For Worms Fenbendazole (Safeguard, Panacur) or Valbazen looks like the most popular: