Reply To: I know adults should take at least 5,000 i.u. per day, and that they can take 50,000 i.u. for some days if fighting the flu. Our 5-year old son, small for his age, takes 1,000 i.u. per day, and 2,000 i.u. once a week. If he gets the flu, what is a safe increased amount to give him to help fight it? Thank you!