Research on the longitudinal impact of using the internet as an information source on patients’ beliefs and medication adherence is scarce. Chronic patients (N = 107) from six hospitals were surveyed to longitudinally explore their online information seeking behavior throughout treatment (i.e., before the consultation about their newly prescribed medication in the initiation phase and after six months in the implementation phase) and how this affects their medication beliefs (concerns and necessity) and medication adherence after three weeks (T1) and six months (T2). Most patients (79%) used the internet. Patients who used the internet before the consultation reported to have more concerns about their medication at T1 and T2 compared to those who did not. Moreover, patients who used the internet throughout treatment valued their concerns higher than the necessity after six months (T2). Patients who used the internet after the consultation reported to be more non-adherent after three weeks (T1) compared to those who did not. Because of the longitudinal nature of this study, we were able to pinpoint in which treatment phase patients’ online information seeking behavior is particular relevant in affecting patients’ beliefs and medication adherence.