Hi Karen,

Metformin dose is 30mg/kg 2x a day. You'll need to use a weight tape or the weight calculation chart (https://ecir.groups.io/g/main/files/Case%20History%20Tools/Tools%20and%20How-To%20Help/Horse%20Weight%20Calculator.xlsx) to figure out his weight and convert from pounds to KG.  

You can give Metformin with other meds and you'll want to recheck his insulin 7 - 10 days after starting Metformin to see if he's responding to it.  If he does he will stay on it at least until he reaches his ideal weight and his ACTH is in the lower end of the reference range. You do want to increase your dosage of Prascend now even with adding Metformin in.