What a solution is
A solution is a single, clear, uniform liquid in which the drug — the solute — is dissolved molecule-by-molecule in a solvent. Because the drug is already in solution, there is nothing to shake, settle or dissolve in the gut: it is the simplest liquid dosage form and often the fastest to act. A solution is also the easiest to dose flexibly — you can pour exactly the volume a small child needs, which is why so many paediatric medicines are liquids.
The big constraint is simple: the drug must actually dissolve, and stay dissolved, at the concentration you need. The whole craft of liquid formulation is built around the drug's solubility — and what you can do when it is not high enough.
Choosing the vehicle
The liquid the drug rides in is the vehicle. Water is the default vehicle — cheap, safe, tasteless — so most oral liquids are aqueous. But many drugs are poorly water-soluble. The first lever to pull is cosolvency: blending water with a water-miscible solvent such as ethanol, glycerin or propylene glycol to make a friendlier home for a less polar drug.
- Confirm the target concentration (e.g. mg per 5 mL) and check it against the drug's water solubility.
- If water alone falls short, add a cosolvent stepwise and watch the drug dissolve; record the smallest fraction that works.
- Keep cosolvent levels as low as the drug allows — ethanol and propylene glycol both have taste and safety limits, especially for children.