Abstract: Due to the small numbers of trials in each category, the limited quantitative data available and the marked differences between trials in terms of participants, interventions and outcome measurement, we felt that pooling of the results was inappropriate. We included 29 trials (19 in adults, 10 in children) involving 4835 people (3799 adults and 1036 children). All studies were placebo-controlled RCTs. However, assessment of the risk of bias of the included studies was limited by poor reporting, particularly for the earlier studies. In the adult studies, six trials compared antitussives with placebo and had variable results. Three trials compared the expectorant guaifenesin with placebo; one indicated significant benefit, whereas the other two did not. One trial found that a mucolytic reduced cough frequency and symptom scores. Two studies examined antihistamine-decongestant combinations and found conflicting results. Four studies compared other combinations of drugs with placebo and indicated some benefit in reducing cough symptoms. Three trials found that antihistamines were no more effective than placebo in relieving cough symptoms. In the child studies, antitussives (data from three studies), antihistamines (data from three studies), antihistamine-decongestants (two studies) and antitussive/bronchodilator combinations (one study) were no more effective than placebo. No studies using expectorants met our inclusion criteria. The results of one trial favoured active treatment with mucolytics over placebo. One trial tested two paediatric cough syrups and both preparations showed a 'satisfactory response' in 46% and 56% of children compared to 21% of children in the placebo group. One new trial indicated that three types of honey were more effective than placebo over a three-day period. Twenty-one studies reported adverse effects. There was a wide range across studies, with higher numbers of adverse effects in participants taking preparations containing antihistamines and dextromethorphan. The results of this review have to be interpreted with caution because the number of studies in each category of cough preparations was small. Availability, dosing and duration of use of over-the-counter cough medicines vary significantly in different countries. Many studies were poorly reported making assessment of risk of bias difficult and studies were also very different from each other, making evaluation of overall efficacy difficult. There is no good evidence for or against the effectiveness of OTC medicines in acute cough. This should be taken into account when considering prescribing antihistamines and centrally active antitussive agents in children; drugs that are known to have the potential to cause serious harm.

Plain-language summary: We identified a broad range of studies of different types of preparations used at different dosages in both adults and children. The evidence is current up to March 2014. We found no good evidence for or against the effectiveness of OTC medications in acute cough. Nineteen studies reported adverse effects of these medications and described infrequent, mainly minor side effects such as nausea, vomiting, headache and drowsiness. The results of this review have to be interpreted with caution because the number of studies in each category of cough preparations was small. Many studies were poorly reported making assessment of risk of bias difficult. While all studies were placebo-controlled randomised controlled trials only a minority reported their methods of allocation and randomisation and there was lack of reporting of blinding of outcome assessors and whether cough outcome measures were validated. In addition, studies supported by pharmaceutical companies or other providers were more likely to have positive results. Studies were very different from each other in terms of treatment types, treatment duration and outcomes measured, making evaluation of overall effectiveness of OTC cough medicines difficult.