A prescriber writes "syrup" and a patient hears "the liquid one". Between those two points sit three quite different formulations, and which one a molecule ends up in is decided long before anything is filled.

Syrup: Dissolved and Stable

In a true syrup the active ingredient is dissolved. Every millilitre is identical to every other, which makes dosing forgiving — a bottle that has not been shaken still gives the right dose. The constraint is solubility: the molecule has to dissolve, and stay dissolved, for the whole shelf life.

Suspension: Dispersed, and Shaken

A suspension carries the active as fine particles held in the liquid rather than dissolved in it. It opens the door to molecules that will not dissolve, and to higher strengths in a smaller volume. The trade is that the instruction to shake well before use is not a formality — an unshaken bottle gives a weak dose at the top and a strong one at the bottom.

Dry Syrup: Water Added Last

Some molecules are perfectly stable dry and degrade steadily once wet. A dry syrup ships as a powder and is reconstituted at the point of use, which is why these carry a short in-use period — often seven to fourteen days — quite separate from the shelf life printed on the carton.

Conclusion

The right form is the one the molecule allows and the patient can use correctly. A more elegant formulation that gets shaken badly is worse than a plain one that does not need shaking at all.