Epidemiological studies reporting positive associations between smoking and oral contraceptives (OC) and Crohn’s disease first appeared in the mid-1980s. Subsequently neither association has been found to be especially strong (relative risks around 3 for smoking and 2 for OC) and there continues to be uncertainty as to whether or not these are causal associations.1 Smoking and OC use also confound one another, with OC use being more common in smokers.

In contrast to the association between Helicobacter pylori and duodenal ulcer, it is not possible to test the reversibility of the associations by randomised trials. A less rigorous test of causation is to observe the relations between these factors and prognosis of Crohn’s disease as Timmer et al have done. Examining prognosis in the context of a controlled trial has several attractions—prospective data recording, minimal problems of misclassification and unbiased assessment of outcome.

Their twofold increase in relapse rate for current smokers is consistent with earlier retrospective studies. The relapse rate is less well-established in ex-smokers. Retrospective assessment of ex-smoking presents …