Use of health care services by people with SUD is high. Our results for ED and hospitalizations episodes are in line with the results of previous studies [25]. At the same time, our study gives a better understanding of the number of contacts with general practitioners by people with SUD. People in treatment for SUD have between 1.6 and 1.9 times more contacts with a GP than their comparators, except for people in treatment for opiate use disorders who have on average 2.6 times more contacts with their GP than their respective comparators. The reason for this higher frequency might be that many of them are on substitution treatment for which they have to consult a GP. For all other considered health service providers, people with alcohol use disorders had more frequent contacts than people with disorders related to street drugs and the comparators. This last observation is in contrast to findings from a previous study where people with alcohol use disorders were found to use less health care services than the general population [30].

Overall, people with SUD had a higher use of primary care than acute health care, which is in line with findings from the few number of studies who reported on this [31, 32]. Compared to the comparators, the differences in use of health services between people in treatment for disorders related to cannabis, stimulants and crack/cocaine are minimal for contacts with the GP, and not significant for hospitalizations and contacts with psychiatrists. Only people in treatment for cannabis use disorders have significantly less contacts with the ED than the comparators compared to people with disorders related to stimulants and crack/cocaine.

Interestingly, as shown by the Figs. 1, 2, 3 and 4, there is little difference in the number of contacts with the specific health service providers over time within each category of SUD. Indeed, the use of health services remains quite stable in the five years before and after the moment people with SUD entered into treatment. Of course, this does not provide insights in individual variation over time, but it remains interesting to see that this level of health care use was that stable. We could not find other studies to support or contradict this finding.

One reason for the high use of general health services might be that the data is based on the first episode in TDI between 2011 and 2014, with between 36.6 % (cannabis) and 80.7 % (opiates) of the cases who had been already in treatment for SUD before, meaning that they might have had already serious substance use related and general health care problems before the day on which they were registered in TDI. We also acknowledge the fact that the proportion of people with hospital admissions in the months after start of treatment for alcohol cases (Fig. 4) shows a slower decline than for other substance categories, although at this time we cannot provide a good explanation for this.

A previous study concluded that the higher use of health care services by PWUD, compared to comparators from the general population, reflects the greater need for treatment, but that it does not necessarily represent good health-care access and that this might reflect a pattern where primary and preventative health care is poor and PWUD rely mainly on unplanned health care [25]. The current study gives a more nuanced picture. Indeed, PWUD make more use of ED and are more admitted to hospitals, but also the use of GPs is significantly higher than for comparators. This higher use of primary health care could underline the presence of high needs, as does the substantial higher use of psychiatric care. At the same time, the current study cannot conclude that access for PWUD is sufficient or that existing care services answer the needs of PWUD, since we only analyzed the use of health care and not outcome of care. For instance, postponing to seek and receive health care is likely to increase the risk of developing complications [33]. Even if PWUD attend more often than comparators a GP, it might still be that they wait much longer to seek help, which might result in more intense use of health care services.

The current study has several limitations, some of which have already been mentioned in previous articles [1, 6, 28]. Three out of four episodes in TDI are registered with a NIN. This means that for one quarter of all episodes it was impossible to identify the person with SUD and to extract their data from the IMA-database. People who are registered without NIN are known to be more in treatment in a psychiatric unit of a general hospital, in treatment for opiates, in treatment in Wallonia and to be non-Belgian [34], but as shown by a previous study, there is little evidence that patient characteristics have a major impact on the registration with a NIN [34].

Moreover, outreach services and harm reduction initiatives are not covered by TDI, nor are GPs who are considered to play a major role in detecting and managing SUD directly [26]. This means that people with SUD who are only treated by GPs are not included in the database. However, for all people registered in TDI, data about their consultations to GPs are registered in IMA. As a result the exact denominator of all people who are in treatment for SUD in Belgium is unknown, but the proportion of them who are registered in TDI can be understood as the large majority. Further research will look into this denominator. Because health insurance is mandatory in Belgium, for almost all people who are registered in TDI with a NIN, data are available in IMA. As such, the reported results give a reliable understanding of the use of services for general healthcare problems by people with SUD in Belgium who are registered in TDI with a NIN.

Finally, the use of register data for a vast and diversified group of people in treatment for SUD allows a better understanding of the number of contacts with health care service providers. However, the relatively limited evolution over time, and the fact that almost 60 % of the cases had been in specialized treatment for SUD before, supports the idea that the development of SUD is a long-term process. Further research should thus take a longer time period into consideration to describe earlier phases in the use of general health care services leading to specialized treatment of SUD.