When you look at over-the-counter remedies, you will see a mixture of one or more of the following items: a decongestant, antihistamine, cough suppressant and expectorant. Some preparations may also contain a pain/fever reliever as well.
None of these ingredients can "cure" a viral cold illness. However, they may help relieve symptoms to make the person taking them feel more comfortable. Recently, the FDA has recommended that children under 6 years of age not be given any over-the-counter (OTC) cough/cold medications due to the risk of side effects. There are still some prescription cough/cold medications available for this age group, however. Whether or not to prescribe a cough/cold medication is usually an individual decision by your pediatrician based on your child's level of illness.
Decongestants are useful for clogged up noses. Phenylephrine is the major decongestant found in OTC medications. Pseudoephedrine is also a common decongestant, though its ability to be used as an ingredient in making amphetamine has made it a closely regulated drug. This is the ingredient that makes you have to sign a release at the pharmacy before buying a medication that contains it. Side effects include irritability, jitteriness and hyperactivity.
Antihistamines are useful for allergies and drying up light nasal drainage from infections. These can make children sleepy, however, and you may wish to only use them at bedtime.
The only cough suppressant available in an OTC preparation is dextromethorphan. Cough is a natural reflex to clear the respiratory tract of unwanted items such as mucus or phlegm, and your doctor may not wish to suppress your child's cough during the day.
The expectorant used in cough/cold medications is guaifenesin. It is supposed to help by thinning the mucus and making it easier to cough up mucus or phlegm, though reports vary as to how effective it may be. Drinking lots of fluids when sick with a cough due to an infection may be just as useful.
Do not use cold/cough medications at the same time as allergy medications. Do not combine cold/cough medications that contain fever relievers with Tylenol or ibuprofen without talking to your doctor first.