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30/10/2025, 00:02 Pharmaceutical Solutions & Sterile Dosage Forms
Principle of cosolvation:
The principle 'like dissolves like'. Polar drugs generally dissolve in polar solvents and non-
polar drugs generally dissolve in non-polar solvents.
Generally poor solubility of drugs in a given solvent results from great difference in polarity
between them, the use of co-solvent aims to reduce such a difference by making the
solvating environment less polar. This improves the solubility of the solute.
One key aspect of drug solubilization with the aid of cosolvents is to create a solvent
mixture that closely matches the polarity of the hydrophobic solute.
Example: Digoxin injection contains 40% propylene glycol and 10% ethanol.
Co-solvents are generally less safe than water and the concentration used in an
aqueous solution is limited primarily by their toxicity and cost.
The co-solvent must be non-toxic at the concentrations used, and by the route of
administration.
The commonly used co-solvents are glycerol, propylene glycol, ethanol and Low MWt
polyethylene glycol (PEG 200, PEG 400). They are water miscible organic solvent used
to enhance the aqueous solubility of a hydrophobic drugs.
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