To take medicine internally (e.g., orally). Used medically as 'to take (a medicine) internally'; can also mean oral medication.
医師の指示どおりに薬を内服してください。
Please take the medicine internally as directed by your doctor.
食後にこの内服薬を内服すること。
Take this oral medicine after meals.
この薬は内服すると眠気を催すことがあるので注意してください。
This medication can cause drowsiness when taken, so please be careful.