Abstract Objective Worldwide pharmacists play an increasingly important role in pharmacovigilance. Lareb Intensive Monitoring (LIM) in the Netherlands is a new form of active pharmacovigilance where pharmacists play a key role. Patients using drugs which are monitored are identified in the pharmacy and invited to participate in the active monitoring. Not all invited patients participate. This study aimed to investigate non-response bias in LIM, as well as reasons for non-response in order to identify barriers to participation. Methods The study population consisted of patients who received a first dispensation of an antidiabetic drug monitored with LIM between 1 July 2010 and 28 February 2011. Possible non-response bias was investigated by comparing age, gender and the number of drugs used as co-medication. Reasons for non-response were investigated using a postal questionnaire. Key findings Respondents were on average 4.5 years younger than non-respondents and used less co-medication. There were no differences regarding gender. The main reason for non-response was that information in the pharmacy was lacking. Conclusion Differences between respondents and non-respondents should be taken into account when analysing and generalising data collected through LIM, as this might contribute to non-response bias. The relatively high response to the postal questionnaire, together with the answers about reasons for non-response, show that patients are willing to participate in a web-based intensive monitoring system, when they are informed and invited in the pharmacy.