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A cough medicine (or linctus, when in syrup form) is a medicinal drug used in an attempt to treat coughing and related conditions. For dry coughs, treatment with cough suppressants (antitussives) may be attempted to suppress the body's urge to cough. However, in productive coughs (coughs that produce phlegm), treatment is instead attempted with expectorants (typically guaifenesin, in most commercial medications) in an attempt to loosen mucus from the respiratory tract.
Recently, studies have questioned the efficacy of over the counter cough medicines, particularly when used by young children, yet they continue to be sold and used in large volume. Even though they are used by 10% of American children weekly, they are not recommended in children 6 years of age or younger due to lack of evidence showing effect, and concerns of harm.
Dextromethorphan (DXM) may be modestly effective in decreasing cough in adults with viral upper respiratory infections. It acts centrally in the medulla to raise the threshold for cough stimulus. It is considered to have equal or near-equal anti-tussive potency to that of codeine. In children, however, it has not been found to be effective.
Codeine was once viewed as the gold standard in cough suppressants. There is evidence that it has a similar mechanism to dextromethorphan, and produces effective cough suppression. Some recent placebo-controlled trials have found, however, that it may be no better than placebo for some etiologies including acute cough in children. It is thus not recommended for children.
Noscapine is a cough suppressant that is available as an over-the-counter drug in many countries.
Bromhexine is a mucolytic that acts at the formative stages of mucus formation in the glands within the mucus secreting cells. It disrupts the structure of acid mucopolysaccharide fibers thereby producing less viscous mucus, which is easier to expectorate.
Guaifenesin is an expectorant that is suspected to indirectly increase the output of respiratory tract fluid, thereby improving the flow of less viscid secretions, and promoting the action of respiratory cilia to facilitate the removal of mucus; it is unclear what mechanism contributes most to its expectorant action.
A number of other commercially available cough treatments have not been shown to be effective in viral upper respiratory infections, including in adults: antihistamines, antihistamine-decongestant combinations, Benzonatate, and guaifenesin; and in children: antihistamines, decongestants for clearing up the nose, or combinations of these.
Honey may be a minimally effective cough treatment. However a Cochrane review found insufficient evidence to recommend for or against its use. Honey's use as a cough treatment has been linked on several occasions to infantile botulism and as such should not be used in children less than one year old.
Many natural treatments are used to treat the common cold. However, a 2007 review states that, "Complementary and alternative therapies (i.e., Echinacea, vitamin C, and zinc) are not recommended for treating common cold symptoms; however, ... Vitamin C prophylaxis may modestly reduce the duration and severity of the common cold in the general population and may reduce the incidence of the illness in persons exposed to physical and environmental stresses."
A 2009 review found that the evidence supporting the effectiveness of zinc is mixed with respect to cough, and a 2011 Cochrane review concluded that zinc "administered within 24 hours of onset of symptoms reduces the duration and severity of the common cold in healthy people". A 2003 review concluded: "Clinical trial data support the value of zinc in reducing the duration and severity of symptoms of the common cold when administered within 24 hours of the onset of common cold symptoms." Nasally applied zinc gel may lead to long-term or permanent loss of smell. The FDA therefore discourages its use.
A review of sixteen trials of echinacea was done by the Cochrane Collaboration in 2006 and found mixed results. All three trials that looked at prevention were negative. Comparisons of echinacea as treatment found a significant effect in nine trials, a trend in one, and no difference in six trials. The authors state in their conclusion: "Echinacea preparations tested in clinical trials differ greatly. There is some evidence that preparations based on the aerial parts of Echinacea purpurea might be effective for the early treatment of colds in adults but results are not fully consistent. Beneficial effects of other Echinacea preparations, and for preventative purposes might exist but have not been shown in independently replicated, rigorous randomized trials."  A review in 2007 found an overall benefit from echinacea for the common cold, however further analysis found problems with the interpretations of this review.
While a number of plants and Chinese herbs have been purported to ease cold symptoms, including ginger, garlic, hyssop, mullein, and others, scientific studies have either not been done or have been found inconclusive.
According to the New York Times, at least eight mass poisonings have occurred as a result of counterfeit cough syrup, substituting inexpensive diethylene glycol in place of glycerin. In May 2007, 365 deaths were reported in Panama, which were associated with cough syrup containing diethylene glycol.
The efficacy of cough medication is questionable, particularly in children. A 2008 Cochrane review concluded that "There is no good evidence for or against the effectiveness of OTC medicines in acute cough". In 2001, a meta-analysis indicated that some cough medicines may be no more effective than placebos for acute coughs in adults, including coughs related to upper respiratory tract infections. In 2006, the American College of Chest Physicians published a guideline for whooping cough, a cough that is caused by bacteria and can last for months. The guideline pointed out that available cough medicines are not designed to treat whooping cough or its causative bacterium. Although the efficacy is inconclusive for children over 2 years of age, a number of factors including accidental overdoses and well-documented adverse effects suggested caution in the pediatric prescription of cough syrups and medicines. No over the counter cough medicines have been found to be effective in cases of pneumonia.