[All the citations in this work have been found by the following NCBI MeSH terms (in different combinations): honey, cough, antitussive agents, adverse effects. The NCBI filters used were: systematic reviews, clinical trial.
As additional source for finding a stronger evidence works was the Cochrane Library, as cited in square blanked in the text.]
What is cough briefly
Cough is “a sudden, audible expulsion of air from the lungs through a partially closed glottis, preceded by inhalation. It is a protective response that serves to clear the trachea, bronchi, and/or lungs of irritants and secretions, or to prevent aspiration of foreign materials into the lungs.” (MeSH terms)
The reflex is mediated by (and starts with) stimulation of bronchial vagal terminations that are activated by mechanical, chemical and biological factors. Cough receptors are located in posterior wall of trachea, pharynx and carina while they are absent in distal airways.
Then the recurrent laryngeal nerves bring the irritative signal to the solitary nucleus (NTS) through the inferior cervical ganglium; the next stop id dorsal nucleus of vagus nerve that brings the parasympathetic output to viscera.
So, leaving Physiology texts the entire description of cough phenomenon, it is possible to assure that cough is inhibited by anything could interfere with any of the stops in the coughing process (for example, reducing broncho-laryngeal receptors activation).
Cough therapy: over-the-counter (OTC) antitussive agents
Cough represents one of the most common symptom for which patients seek medical care and yet effective, well-tolerated cough medicines remain a significant unmet clinical need. In fact, probably due to different cough-evoking mechanisms for different diseases, literature lacks in evidences about effectiveness of the most common therapies for cough.
A systematic review on the effectiveness of the most commonly used therapies in different types of cough-associated illnesses (excluding cancer) demonstrate that a more strong research designs are needed to get evidence from cough therapies [Molassiotis A et al, Respir Med, 2010 Jul;104(7):934-44. doi: 10.1016/j.rmed.2010.02.010. Epub 2010 Apr 10].
A Cochrane Database systematic review [Schroeder K et al, 2004 Oct 18;(4):CD001831] shows that there’s no evidence of over the counter (OTC) antitussive agents in cough treatment for adults and youngs. First of all, in adults, codeine is no more effective than placebo in reducing cough symptoms; dextrometorphan showed opposing results; moguisteine was only effective in a small group of patients affected by a particularly strong kind of nocturnal cough. In children, one study showed no more effectiveness of antitussives than placebo.
In addition to absence of evidence for OTC antitussive drugs effectiveness, it is also important to consider their possible collateral effects. For more information see the following: three case reports at Friedrichsdorf SJ et al, J Opioid Manag. 2013 Mar-Apr;9(2):151-5. doi: 10.5055/jom.2013.0156 and Naveed Ahmed et al, Pediatr Rep. 2013 June 13; 5(2): 31–34
A baboon syndrome has been reported in a tree-year boy associated with an antitussive-decongestant containing dextromethorphane and pseudoephedrine HCl [Ozdemir H et al, Turk J Pediatr. 2010 Nov-Dec;52(6):659-61].
The compresence of codeine in most of antitussive drug commonly sold could improve abuse behaviors [Murao S et al, Intern Med. 2008;47(11):1013-5. Epub 2008 Jun 2].
OTC antitussive-expectorant drugs may lean to death (even rarely) infants, as shown by a CDC case report [MMWR Morb Mortal Wkly Rep. 2007 Jan 12;56(1):1-4].
Is honey an eligible ressource for cough treatment?
A single study compared honey to placebo (silan date extract) on nocturnal cough and difficulty sleeping associated with childhood upper respiratory tract infections (URIs), showing a kind of effectiveness for both honey and placebo group in the night of the treatment compared to the night befroe treatment.[Herman Avner Cohen et al, PEDIATRICS Volume 130, Number 3, September 2012]
The World Health Organization identifies honey as an effective demulcement for cough in post upper respiratory tract infections [Naveed Ahmed et al, Pediatr Rep. 2013 June 13; 5(2): 31–34]
Anyway, Cochrane reviews show no evidence of honey effectiveness in treatment of dry, chronic non specific cough [Selamawit Mulholland et al, Cochrane Airways Group, Published Online: 20 JAN 2010, Assessed as up-to-date: 27 OCT 2010, DOI: 10.1002/14651858.CD007523.pub2].
The exact mechanism of honey is not well known, but it is well known than honey reduces cough frequency in children (MD -1,07 with CI from -1,53 to -0,60 at 95%), as well as cough severity (MD -0,97 with CI from -1,47 to -0,46) [Cochrane revision of seventynine studies, only two of which could be included: Paul et al, 2007; Shadkam et al, 2010]
A possible explanation for honey effectiveness in aspecific dry cough
It is proved that sweet and viscous placebo preparation may reduce coughing in frequency and severity, probably by a stimulating action of honey viscous sweet composition on reflex salivation and airways mucus secretion, <>. [Eccles R, Respir Physiol Neurobiol. 2006 Jul 28;152(3):340-8. Epub 2005 Dec 2] This effect could be extended to every sweet viscous preparation, including the one with dextromethorphane.
A solid, strong evidence for honey effectiveness in cough treatment actually lacks: more, appropriately designed randomized double-blind studies are required.