Our database search returned a total of 4290 records. After duplicates were removed, 2824 records were screened on titles and abstracts. This resulted in 242 records for full text screening or in-depth abstract assessment (if only an abstract was available). Of these, 41 records were included in the review. In addition, citation search yielded a further 9 included records. In total, we thus obtained 50 reports or abstracts. As four conference abstracts preceded full-text publications, these abstracts were not required for our analyses. Consequently, our review comprised 46 publications containing data from 45 studies, as different aspects of one study were published in two reports (see flow diagram, Fig. 1) [45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90]. Reasons for exclusion of records were primarily due to missing (separate) data on children or on drug-related hospitalisations (drug-related problems during hospitalisation were often included instead). All records of the eligibility assessment together with the corresponding selection decisions are documented in the Additional file 4.

Flow diagram based on “PRISMA 2020 flow diagram for new systematic reviews which included searches of databases, registers and other sources” [22]. Abstracts for which no reports could be retrieved were also assessed for eligibility. Therefore, in addition to reports, eligible abstracts were also included. This additional information is shown in blue

Categorisation according to the definition of ‘drug-related problems’ revealed that sixteen studies [45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60] were based on the broader concept of ‘adverse drug events’ [13, 24, 26], while 29 studies [61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90] focused on ‘adverse drug reactions’ [13, 27]. Although the studies could clearly be assigned to one of the two categories, the exact definitions still varied between the studies or were not given. Other concepts of drug-related problems were not used.

Subsequent accounting for the second fundamental criterion, the ‘intensive’ or ‘routine’ monitoring methodology, resulted in the four groups of studies: a) Studies on ‘adverse drug events’, ‘with intensive monitoring’, b) Studies on ‘adverse drug events’, ‘based on routine monitoring’, c) Studies on ‘adverse drug reactions’, ‘with intensive monitoring’, and d) Studies on ‘adverse drug reactions’, ‘based on routine monitoring’ (see Fig. 2 and Additional file 5). Within each of these groups, there was further considerable heterogeneity. This concerned general study parameters, study populations as well as methods. Figure 2 provides an overview of the observed aspects of heterogeneity, Additional file 5 contains the detailed study characteristics and descriptive results.

Overview of the heterogeneity found in the 45 studies included

We compiled the results of the critical appraisal in the summary plot (Fig. 3) and in ‘traffic light’ plots (see Fig. 4). The bar chart of the summary plot shows the distribution of critical appraisals based on equal weighting of the 45 studies and thus informs about the overall assessment of the studies. It highlights two major issues: Firstly, all 18 studies with ‘routine monitoring’ [53,54,55,56,57,58,59,60, 81,82,83,84,85,86,87,88,89,90] showed a lack of information on the characteristics of the study design, especially in the description of the study population. Thus, there are serious issues in the reporting. This problem was found also in 20 [46,47,48,