cosolvency

Cosolvency is mixing a second, friendlier liquid into water to coax a stubborn drug into solution — much like adding a splash of alcohol to dissolve a flavouring that water alone leaves cloudy. The blend dissolves what neither pure liquid could manage well on its own for that drug.

It works by lowering the polarity of the medium. Water is very polar and rejects oily molecules; a water-miscible cosolvent such as ethanol, propylene glycol, glycerol or polyethylene glycol reduces the overall polarity so the medium better matches the drug. For many poorly soluble drugs solubility rises roughly exponentially as cosolvent fraction increases, sometimes by orders of magnitude.

Cosolvents are a mainstay of liquid and parenteral formulation, but they carry constraints. They can be toxic or irritant in large amounts, may not be tolerated by every route, and — critically — diluting a cosolvent solution with water (or with blood after injection) can cause the drug to precipitate as its solubility suddenly drops. Cosolvency is therefore often combined with other approaches and used at the minimum effective level.