This study revealed an overall poor vitamin D status in this cohort drawn from an SUD, and predominantly OUD, population in Western Norway. Fifty-seven percent of the cohort had serum vitamin D concentrations below 50 nmol/l corresponding to deficiency, and 19% were severely deficient with serum levels below 25 nmol/l. Although vitamin D deficiency is not uncommon in Norway, the numbers presented in our study are markedly higher than those reported in studies on the general Norwegian population (57% vs. < 40%)^30–32. In particular, the prevalence of severe deficiency is ~ 5 times higher in our study cohort compared to numbers presented in studies on the general Norwegian population (19% vs. ~ 4%)^30–32. These finding are in line with another Norwegian study among heavy substance users reporting a mean serum vitamin D concentration of < 40 nmol/l, well below the threshold of deficiency⁵. Similarly, a descriptive study on methadone-users in the Boston-region of the US found that 36% were deficient in late summer-fall season¹¹. In contrast, case–control studies from respectively Turkey and Australia have reported of more favorable serum concentrations of vitamin D among people with SUD and OUD compared to healthy controls^12,13.

We did not find that substance use patterns, namely severity of substance use nor the consumption of most individual substance classes, were associated vitamin D levels. One exception was found for cannabis where frequent consumption was associated with lower vitamin D levels at baseline (however, without significant time trends). This finding is in line with another study describing lower serum vitamin D levels among heavy cannabis users compared to non-users³³. Additionally, we did not find support of an association of opioid consumption with vitamin D levels in this population. Neither enrollment in OAT, the dose of opioid agonist prescribed, nor the use of non-OAT opioids were associated with vitamin D levels. The absence of consistent associations between substance use or treatment-related factors and vitamin D levels in our analyses suggests that there could be other factors contributing. A multitude of adversities are known to accompany sustained and severe substance use, including a high burden of disease, disability and economic disadvantage. In light of this, we found that the subset of participants in our cohort (8%) whose income stemmed from paid labor had markedly higher serum vitamin D levels compared to those receiving benefits. Employed individuals feasibly have a higher income and lower health impairment compared to the majority that rely on social benefits for income linked to disorders, disability, disease, or unemployment. The overall high prevalence of subjective health-complaints in this population, including fatigue³⁴ and psychological complaints³⁵ could contribute to behaviors that impact vitamin D status negatively, such as inactivity³⁶ and less time spent outdoors exposed to ultraviolet radiation³⁷.

Maintaining adequate vitamin D levels is important for many aspects of health³⁸, and the high prevalence of deficiency poses a potential additional burden to the health of people with severe SUD. Addressing this could have positive outcomes on their health and well-being. In line with this, a randomized controlled trial on supplementation with 50,000 IU vitamin D every other week for 12 weeks showed promising results for several aspects of the health and well-being of people receiving methadone for OAT, including improved sleep quality, reduced psychological symptoms and blood markers of systemic inflammation and glucose tolerance³⁹. As the prevalence of vitamin D deficiency, and in particular severe deficiency, was substantial in our study cohort, we suggest that the time has come to consider implementing targeted policies, including monitoring and/or supplementation in this and similar at-risk populations⁴⁰.