To investigate the time course of medication adherence and some of the factors involved in this process in undocumented migrants with chronic diseases.

Retrospective cohort study.

A big non-governmental organisation in Milano, Italy, giving medical assistance to undocumented migrants.

1918 patients, 998 females and 920 males, with at least one chronic condition (diabetes, cardiovascular diseases (CVDs), mental health disorders) seen over a period of 10 years (2011–2020). Their mean age was 49.2±13 years.

Adherence to medications decreased over 1 year in all patients. This was more evident during the first 2 months of treatment. Patients on only one medication were less adherent than those on more than one medication; at 6 months the percentage of patients with high adherence was 33% vs 57% (p

Interventions to increase medication adherence of undocumented migrants with chronic diseases are necessary, particularly in the first 2 months after beginning treatment. These should be aimed at people-centred care and include more outpatient consultations. Educational interventions should especially be taken into consideration for patients on monotherapy.