The weight ranges for infants taking ibuprofen are 12 to 17 pounds, 18 to 23 pounds and 24 to 35 pounds. Adults should not administer ibuprofen to infants less than 6 months old unless a doctor instructs them to do so, according to MedlinePlus.
In addition to determining where an infant falls in the weight range, adults who administer ibuprofen also must take into account the medicine's strength and type, which is listed on the label, notes MedlinePlus. For infant drops labeled as 50 mg/1.25 mL, the appropriate dosage is 1.25 mL for a baby weighing 12 to 17 pounds. For an ibuprofen liquid labeled as 100 mg/1 teaspoon, the correct dosage is 1/2 teaspoon for the same size baby.