salt formation
Salt formation is giving a drug an ionic partner so it dissolves more easily — pairing an acidic or basic drug with an oppositely charged counterion, the way table salt is just sodium paired with chloride. The drug becomes a salt that water embraces far more readily than the neutral parent.
Most drugs are weak acids or weak bases. A basic drug reacted with an acid (for example hydrochloric acid) gives a salt such as a hydrochloride; an acidic drug reacted with a base gives a salt such as a sodium or potassium salt. These ionic salts dissolve and dissolve faster than the un-ionised free acid or free base, because charged species are strongly hydrated, and they also tend to be crystalline solids that are easier to purify, handle and manufacture.
The counterion is chosen during salt selection by weighing solubility, dissolution rate, crystallinity, melting point, hygroscopicity, stability and physiological acceptability. Salt formation only works for ionisable drugs, and it does not change the intrinsic solubility of the neutral molecule that ultimately reaches the absorption site — in the gut a salt can convert back toward the free form, so the benefit is greatest on dissolution rate and on making a manageable solid.
Salt formation changes the solid that goes into the bottle and how fast it dissolves; it does not create a new active molecule. Once dissolved and absorbed, the drug acts identically — the counterion is just along for the ride and is usually pharmacologically inert.