Why it's being given. Your question has many aspects. If you're asking the physical administration, the answer is likely found in mosby's nursing. If you are asking about why to give or total dosage, a lot goes into the decision: respiratory status, renal function, heart function, electrolyte/acid/base status, and the desired goal. It is an effective and frequently used medication often used for complex situtations.
See below. Intravenous administration can lead to same side effects as oral administration including electrolyte disturbances like low sodium, low pottassium. It can also lead to dehydration more quickly than oral, if not carefully dosed. Rarely allergic reactions occur like any other medications.