What is the correct dose for my 12 year old who weighs 113 lbs for zithromax?

Adult dosing. Once children reach above a certain weight, the regular adult z-peak or tri- pak for three days is fine for zithromax.
Varies by illness. Zithromax comes in different liquid concentrations and comes in different tablet concentrations. The dosage will vary based on the type of illness. Ear infections and pneumonia may need 5 days treatment. Sinusitis may need 2 rounds of 5 days each treatment. Sexually transmitted diseases may be treated with one large dose on one day. Few other illnesses can be treated one large dose 1 day.