Isotonicity
Isotonicity describes a solution that exerts the same effective osmotic pressure as plasma, so that cells in contact with it neither swell nor shrink when it is injected.
Tonicity is the effective osmotic pressure a solution exerts across a real cell membrane, and it depends only on solutes that cannot cross that membrane. Osmolality, measured in milliosmoles per kilogram of solvent, counts every dissolved particle whether or not it can cross, which is why the two quantities diverge. Human plasma sits at roughly 275 to 295 milliosmoles per kilogram. A solution matching that with impermeant solutes is isotonic; a solution of a freely permeable solute such as urea can be iso-osmotic on a meter and behave as hypotonic in the body.
The reference fluids make the arithmetic concrete. Sodium chloride at 0.9 percent dissociates into two particles and comes out near 308 milliosmoles per litre, close enough to plasma to be treated as isotonic. Dextrose at 5 percent measures around 278 milliosmoles per litre, but the glucose is metabolised after infusion, so it behaves as free water once in the body. Water for injection given straight into a vein is grossly hypotonic and haemolytic.
In a formulation, tonicity is set deliberately with a tonicity agent, most often sodium chloride, glycerol or mannitol, and that choice interacts with everything else in the vehicle. Hypertonic solutions injected subcutaneously sting and irritate, and markedly hypotonic ones given intravenously cause haemolysis, so tonicity is a tolerability constraint as much as a safety one.
The mistake that surfaces most often is conflating osmolality with tonicity. A second, more practical error is assuming dilution moves a solution toward physiological tonicity: diluting a concentrated product with plain water pushes tonicity below plasma rather than toward it, and diluting an isotonic product with an isotonic vehicle changes concentration without changing tonicity at all.