Observational studies and "the eye of the beholder"

Nwaru et al. [1] report a dose-related association between short-acting β2-agonists (SABA) and poorer clinical outcomes (exacerbations, emergency department visits, hospital admissions and mortality) in asthma. Acknowledging that observational studies cannot establish causality, they conclude: “Regardless of whether there is a causal effect of SABA use and these adverse effects, or if they are mainly a marker for more severe asthma and/or a reflection of the frailty of the patients, increased use of SABA should alert clinicians to monitor these patients more closely.” These are important conclusions that should inform clinical care and be incorporated into international guidelines.