syrup
A syrup is the thick, sweet liquid medicine you probably remember from childhood — a spoonful of something heavy and sugary that slides down easily. Pharmaceutically, it is a concentrated aqueous solution of sugar (usually sucrose) in which a drug is dissolved, designed so that an oral medicine tastes pleasant and pours smoothly.
The high sugar concentration does more than sweeten. A simple syrup is around 85% w/v sucrose, close to saturation, and that near-saturated sugar level lowers the available water (water activity) so much that most bacteria and moulds cannot grow, giving the product a degree of self-preservation. The viscous body also coats the tongue and reduces perceived bitterness of the active ingredient.
Because syrups are aqueous, a drug must be water-soluble to be formulated this way; poorly soluble drugs are better suited to a suspension or an elixir. The heavy sugar load is also a real drawback for diabetic patients and for long-term dosing, so sugar-free syrups using sorbitol or other polyols, plus an added preservative, are now common.
Paediatric paracetamol (acetaminophen) syrup lets a parent measure a small, accurate liquid dose for a child who cannot swallow a tablet, with cherry flavour and sucrose masking the drug's bitterness.
A sweet aqueous solution turns an unpleasant drug into something a child will actually take.
A syrup that contains enough sucrose near saturation can be largely self-preserving, but once diluted or once a sugar-free polyol base is used, an added preservative becomes essential.