Abstract: We included 40 studies involving 1717 participants. The quality of the included studies was largely poor and, with a few exceptions, these comprised of mainly small, inadequately reported studies. When comparing treatment with a single antibiotic to a combined antibiotic regimen, those participants receiving a combination of antibiotics experienced a greater improvement in lung function when considered as a whole group across a number of different measurements of lung function, but with very low quality evidence. When limited to the four placebo-controlled studies (n = 214), no difference was observed, again with very low quality evidence. With regard to the review's remaining primary outcomes, there was no effect upon time to next exacerbation and no studies in any comparison reported on quality of life. There were no effects on the secondary outcomes weight or adverse effects. When comparing specific antibiotic combinations there were no significant differences between groups on any measure. In the comparisons between intravenous and nebulised antibiotic or oral antibiotic (low quality evidence), there were no significant differences between groups on any measure. No studies in any comparison reported on quality of life. The quality of evidence comparing intravenous antibiotics with placebo is poor. No specific antibiotic combination can be considered to be superior to any other, and neither is there evidence showing that the intravenous route is superior to the inhaled or oral routes. There remains a need to understand host-bacteria interactions and in particular to understand why many people fail to fully respond to treatment.

Plain-language summary: The review included 40 studies with 1717 people with cystic fibrosis. Studies compared intravenous antibiotics with placebo (dummy drug with no active medication) and also one antibiotic compared to two antibiotics given together. Specific antibiotic combinations were also compared as were intravenous antibiotics with antibiotics that were breathed in (inhaled) and antibiotics that were swallowed (oral). The studies lasted from three to 15 days, although most of the studies lasted for two weeks. In the comparison between those people who were given just one antibiotic and those who were given two, it appeared that those receiving two antibiotics experienced a greater improvement in lung function, but when limited to only those studies that included a dummy drug, we did not see any difference. There was no effect upon the amount of time until the next exacerbation, weight, or adverse effects. No combination of antibiotics was any better than any other. The outcomes for people were the same irrespective of whether they were treated by intravenous, oral or inhaled antibiotics. None of the studies reported on quality of life. The quality of the included studies was often poor and many were not properly reported. Some studies included volunteers more than once which made comparing treatments difficult. It was also often difficult to decide from the information given how well the studies were carried out - particularly with respect to how volunteers were chosen and whether the volunteers or doctors could tell which treatment they were being given.