Many of the OTC cough syrups owe their decongestant and antitussive effects to the antihistamines they contain.
Anithistamine group of drugs, while reducing the production of mucus, also usually cause dryness of the affected mucous membranes, which is counter-productive to healing and can cause local pain as well.
Paracetamol and other analgesics lead to a lowering of the accompanying fever in various respiratory infections.
However, these two types of drugs exert these general actions only and they also do not show any organ-specificity. So, they act on all tissues and organs in the body, giving rise to many unpleasant side-effects.
Doctors often advise such cough syrups along with the antibiotics and other drugs prescribed by them.

I feel cough syrups such as these are wholly unnecessary. Some of them, those that contain phenylephrine for instance, are actually quite harmful to the heart and codeine can lead to addiction!

There are other, alternative cough syrups from the stables of Herbal Medicine, which it makes better sense to use, therefore.

Herbs have varied actions on the body; some reduce production of mucus in the respiratory tract, while others aid its removal; yet others help by their decongesting effect. Many herbs exhibit an organ-specifity, like Pulsatilla nigricans for the Nostrils and Spongia for the pharynx.
Many herbs have a natural antipyretic action as well.
All this is generally unknown, even to doctors, as Herbology is not one of the subjects taught in Medical College.

There are many herbal combinations possible, with each herb having a specific healing action on the respiratory tract, depending upon a patient's individual requirement. It would definitely be more logical to use these!

Besides, using herbs in the form of dilutions removes their possible toxic actions, so no side-effects would accrue from their use, which is an added advantage!

I have been using such combinations for several years now and feel that they have a lot to offer in the therapy of respiratory infections and allergies, and in treating disorders of other organs and tissues as well.