The dosage of Megapen Kid Tablet for a 15-year-old should be determined after a medical assessment. The correct dose depends on her weight, the type and severity of the infection being treated, and her overall health. Antibiotics should never be self‑administered, as incorrect dosing can cause side effects or antibiotic resistance. Please consult a paediatrician, who will prescribe the appropriate strength and frequency tailored to your daughter's needs.