[johnep]

There are only a few active ingredients to a cough syrup;
1) Demulcents/humectants to keep the throat moist. Examples are sugar syrup, honey, glycerin.
2) Expectorants. These increase mucus production leading to thinner secretions allowing easier expectoration. Used to be ammonium salts, mainly ammonium chloride, ipecac. Now guafenesin is standard.
3) Cough suppressants. Classic was opiods such as morphine and codeine.
Dextromethorphan used today or though opinions vary on efficacy.
4) Antihistamines. These tend to dry up secretions and have replaced atropine. The older ones have sedative properties which can be a benefit at night.
5) Decongestants- Subject to a big row at the moment and looks as if pseudoephedrine will be replaced by phenylephrine.

Cough syrups are just permutations of these ingredients.
johnep