Ivermectin dosage is based on body weight. I take it in 10MG or 5MG tablets (my dosage is 17.5-20MG for 190 pounds weight .). The horse paste comes in a tube with a plunger marked in body weights starting with 250 pounds with five little notches between each 250 pound mark; i. e. 50 pounds/notch). Other tubes may differ.
My horses weighs close enough to 200# that it isn’t worth worrying about the difference. So four notches.
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