What if PPI users are more likely to have some other health issues compared to other patients due to some unobserved variable (e.g. lifestyle, strength of the p

What if PPI users are more likely to have some other health issues compared to other patients due to some unobserved variable (e.g. lifestyle, strength of the physiological reaction to stress, genetic factors affecting both the likelihood of becoming a PPI user as well as the likelihood of having other health issues)? Oh, man... correlational studies... shame they didn't directly link to the article, too.