Salt form

Salt form refers to the version of a drug molecule in which its basic or acidic group is paired with a counter-ion — a partner such as a chloride, sulfate, fumarate, or maleate. The result is an ionic compound: typically a crystalline solid that is more water-soluble and more stable in storage than the uncharged freebase equivalent.

Most psychoactive substances are weak organic bases, meaning they carry a nitrogen group capable of accepting a proton. Pairing that base with an acid yields the corresponding acid-addition salt. Cocaine hydrochloride and MDMA hydrochloride are everyday examples; the word after the hyphen names the counter-ion.

Because the counter-ion contributes its own mass, equal weights of a salt and its freebase do not contain equal amounts of active compound. The ratio — sometimes called the base-equivalent or salt-correction factor — differs by compound and by which counter-ion was chosen, and it matters whenever doses from different sources are being compared.

How it works · its role

The chemical difference between a salt and its freebase is one of protonation: the salt carries a formal positive charge on its nitrogen (or, for acidic drugs, a negative charge), stabilised by the counter-ion. In water — including stomach acid and blood plasma — the two forms coexist in a pH-dependent equilibrium governed by the compound's pKa.

This balance has practical consequences for absorption. The ionised salt form is more water-soluble but crosses lipid membranes poorly; the neutral freebase passes through cell membranes more readily. As the freebase is pulled into tissues, more of the ionised form converts to replace it, so the ratio at the absorption site shifts continuously with local pH.

Different counter-ions also alter physical properties beyond solubility: melting point, crystal habit, hygroscopicity, and vaporisation temperature all shift with the choice of salt — which is why pharmaceutical chemists often screen many salt forms before committing to one for a product.

Relevance to substances & effects

The salt vs. freebase distinction shapes how and where a substance is used, and it appears throughout these pages. Cocaine is the clearest example: cocaine hydrochloride — a water-soluble salt — is suited to insufflation or injection, while cocaine freebase (crack) is smoked, because the neutral form volatilises at a lower temperature without decomposing. The same molecule; two different salt states; markedly different onsets.

Amphetamine is dispensed in several salt forms — sulfate and aspartate among them — and the choice of salt influences formulation stability and release characteristics. Many substances on these pages are listed by their pharmaceutical salt name on packaging, even when the dosing literature was established with a different form. When reading dose figures, check whether the value refers to the salt or to the active base — the two are not interchangeable without correction.