question about equine ivermectin

[catmom5]

question about equine ivermectin -
I did manage to get my boy to eat a walnut with the apple flavored ivermectin on it. He is looking much better and I've not seen him itching. Is it necessary to give him a second dose?
Also, how long is it good for when refrigerated?

thanks

[CritterMom]

Once per week for up to three weeks as needed. I suspect that if kept in an airtight well marked container in the fridge it would keep quite a while...