As the eNotes guide linked below indicates, administration of medications orally may not be appropriate if the medication in question does not absorb into the gastrointestinal tract as well as if it is administered intravenously or through the rectum in suppository form.
Certain medications are best administered through different means other than or in addition to orally. Many antibiotics, for instance, are best administered both through injections and orally, as the injected form will begin to take effect quicker, while subsequent oral doses provide necessary maintenance to ensure that the bacteriological infection does not recur. This is often the case when a traveler is suffering from serious gastroenteritis in certain regions of Asia or Africa. Also, if the patient is having difficulty keeping liquids or food down -- in effect, is regurgitating anything consumed orally -- then medications will not be administered through the mouth, but through suppository or intravenous routes.
The means through which medications are administered is entirely dependent upon the individual patient, symptoms, and medication prescribed. Some simply work better or faster if injected or administered intravenously. Others work fine administered orally depending upon the patient's ability to keep substances from regurgitating and the medication's ability to pass through the digestive tract.