Cough syrups are proprietary mixtures of ingredients that target cold symptoms—and patient expectations. It's hard to analyze the effect of any one ingredient because of the power of placebo effects and because a cough can be caused by many factors—a cold, exposure to irritants, an allergy, residual inflammation from a viral infection, a mucus drip. Sometimes coughing itself causes more coughing, by irritating the airway.

1. Some important cough syrup ingredients: Sugar and flavoring agents—is good or bad taste more effective?—and pretty colors. Personally, I think red works best.
2. Expectorants (like guaifenesin). These are things that make you spit and are said to loosen mucus or increase mucus flow.
3. Antihistamines. Cough medicines use only older ones that cause sedation or act as drying agents. Probably the side effects provide the value, if any, and the blocking of histamine effects is irrelevant.
4. Decongestants. Pseudephedrine is pretty much the only one left on the market. Its side effects include sleeplessness, jitteryness, and agitation.
5. Cough-suppressants like dextromethorphan, which depress the central cough reflex.
6. High-viscosity agents that protect the cilia that line the respiratory system. Here's where my mother's honey and butter remedy comes in (see main text).

Many of these ingredients came into use before medications were required to demonstrate that they were effective. They are still in use more or less out of habit, or perhaps in the belief that they are at least harmless (not necessarily true).