Saline is a sterile mixture of water and sodium chloride (table salt) that has the same water to salt ratio as blood plasma. It is frequently used in medicine, usually as intravenous solution to treat or prevent dehydration in patients. Saline is preferable to water as it will not draw water out of more salty tissues in the body through osmosis, therefore preserving the blood chemistry of the patient. However, patients given saline must be carefully monitored for their urine output as a constant flow of saline without urination will surely lead to edema.
Saline can also be used as a placebo as injecting it into a patient will have no ill effects. Cuddy pulled this trick on House when Stacy left when House, suffering from the psychological pain from the break-up, demanded a shot of morphine.
House once threatened a small child with in intramuscular injection of saline which, although harmless, would have stung. House did it so that the mother of the child would no longer threaten taking her child to the doctor as a form of discipline for other misbehavior.