In medicine saline is a solution of sodium chloride in sterile water, usually for intravenous infusion. Sodium chloride (NaCl) is ordinary salt. Saline solutions are available in various concentrations for different purposes.

Normal saline is the solution of 0.9% w/v of NaCl. It contains 154 meq/l of Na and Cl and has about the same degree of osmolality as blood (referred to as isotonic), about 300 mosm/l. Normal saline (NS) is therefore used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed severe dehydration. Normal saline is typically the first fluid used when dehydration is severe enough to threaten the adequacy of blood circulation and is the safest fluid to give quickly in large volumes.

Other concentrations of saline are frequently used for other purposes, such as supplying extra water to a dehydrated patient or supplying the daily water and salt needs ("maintenence" needs) of a patient who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems, intravenous solutions with reduced saline concentrations typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride. As the molecular weight of dextrose is greater, this has the same osmolality as normal saline but contributes less sodium to the circulation. Because dextrose monohydrate (MW 198 in contrast to MW 180 for glucose) is the commercial form of detrose used in these preparations, 5% dextrose actually contains only 4.5 g/l of glucose.

Concentrations commonly used include

1. Half-normal saline (0.45% NaCl), often with "D5" (5% dextrose), contains 77 meq/l of Na and Cl and 4.5 g/l glucose.
2. Quarter-normal saline (0.22% NaCl) has 39 meq/l of Na and Cl and always contains 5% dextrose for for osmolality reasons.
3. Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.

The amount of normal saline infused depends largely on the needs of the patient (e.g. ongoing diarrhoea or heart failure) but is typically between 1.5 and 3 litres a day for an adult.