Dextromethorphan, usually in the form of Dextromethorphan Hydrobromide, is the only pharmaceutical cough supressant available in the USA over the counter. (There are probably a number of herbal remedies and a few local anaestics such as menthol, but these are not true cough supressants). It provides suppression of the cough reflex directly in the central nervous system for a period of several hours.
I know that many people reading this may think of the idea of using DXM to suppress coughs in the same way they would think about reading Playboy for the articles. And while it is true that DXM's rather strange and dangerous intoxicating effects may overshadow its use as an antitussive, its use for suppressing coughs is quite important.
Although dextromethorphan is indeed a synthetic opioid, and it has a similiar cough suppressing effect, it is not considered a narcotic, that is, its route of action is different from that of codeine or morphine. According to the DXM FAQ :
Sigma receptors are evidently involved in this pathway (42,49,55,56). This may be a direct involvement - sigma activation may directly inhibit the cough reflex signals - or it may be an indirect one. The cough suppressant effect of opiates (such as codeine) is not related to the same effect of non-opiate morphinans like DXM (49); instead, it seems to be governed by traditional opiate receptors (mu, kappa, or delta).
This perhaps doesn't mean anything for those of us not schooled in the difference between the four types of opiate receptor, and who haven't fully delved into the mysteries surrounding the Sigma opiate receptor, but the upshot of all this is that, unlike Codeine, at the recommended dosages for cough supression, DXM does not cause drowsiness, analgesia constipation, and has no potential for abuse.
DXM is usually sold in a syrup form, usually mixed in with Guaifenesin, an expectorant, or drug that causes mucus to become less thick. The combined effects of these two drugs is to make the user cough less, but makes the coughing they do perform more productive.
The dosage of DXM is usually somewhere between 10 and 20 milligrams every four hours. Note that since psychoactive effects don't start until around at least 50 milligrams, it is quite possible to use DXM to relieve a cough with no impairment of function (although driving or operating machinery is probably not a good idea).
The exact mode of action of DXM is hard to explain. My own experience seems to show that it causes a reduction in both the subjective and objective urge to cough. While it is not analgesic at these dosages, it does seem to cause a decrease in the amount of irritation that the cough reflex subjectivly causes.
I went to the Student Health Service at Portland State University yesterday, complaining of some upper respiratory symptoms. The nurse said I didn't have either bronchitis or pnuemonia, and that I just needed some rest. She gave me a bottle of generic Robotussin, even though I hadn't complained of a cough. However, not wanting to look a gift horse in the mouth, I took it home and took it at slightly greater then recommended dosages. The Guaifenesin has really helped clear out my throat, and the DXM has greatly reduced the constant irritation that my throat was giving me.
So, to summarize: Dextromethorphan is a safe, effective and side effect free way to reduce both the objective symptoms and subjective discomfort of a cough.