Opioid addiction is multifaceted with significant health implications for individuals (1, 2). Opioids, which include naturally occurring, semi-synthetic and synthetic substances, bind to opioid receptors in the brain and produce sedative, analgesic and euphoric effects (3, 4). These drugs are commonly used to treat severe pain, including cancer-related pain (5). The US Drug Enforcement Association (DEA) classifies opioids according to their medical use, potential for abuse and risk of dependence. Well-known opioids include medically used substances such as morphine and codeine, which vary in their addictive potential, and non-medically used drugs such as heroin (6).

Opioid abuse is both a psychological and a physical condition (1, 2). It can result from a number of factors, including genetic predisposition, stress, prolonged use of opioid analgesics, and an individual's circumstances and environment. These factors, combined with the increasing availability of illicit opioids, have contributed to the opioid epidemic, particularly in the US (1, 7).

Since the 1990s, the development of more opioid-based drugs by pharmaceutical companies, together with effective advertising campaigns and low costs, has led to a significant increase in prescription rates. Patient privacy laws and a lack of communication across state borders prevented the monitoring of prescription quantities per patient, facilitating the sale of excess opioids on the black market (8–14). The increasing availability of opioids and their high potential for addiction led to a sixfold increase in opioid-related deaths in the US between 1999 and 2017, surpassing HIV-related deaths during the AIDS epidemic (12–15). By November 2017, the annual cost of the opioid crisis in the US had reached $ 504 billion, underscoring its status as a national problem (13, 16).

Similarly, opioid use and prescribing have increased substantially in many European countries since 2004, although not to the same extent as in the US (17). According to a report of the International Narcotics Control Board in 2022, Western and Central Europe ranked second in the world in terms of “defined daily doses (S-DDD) of opioid analgesics per million inhabitants per day” at around 8,500 in 2021 (18). Almost all European countries have experienced an upward trend in prescribed opioid use since 2008, with Germany and Austria showing particularly high rates (19, 20). Projections from addiction treatment centers indicate that the number of opioid addicts in Germany was 166,294 in 2016, suggesting a consistent trend. The rate of opioid addiction varies between the German federal states, with Bavaria's rate estimated at 1.3 per 1,000 inhabitants, placing it in the middle range of the federal states (21).

Despite the well-known effects of opioid addiction on general health, such as constipation, cognitive problems and sleep disorders, oral health is often neglected (22). Drug abuse significantly alters behaviors and lifestyle, leading patients to de-prioritize dental health. Poor oral hygiene can lead to caries, periodontal disease and tooth loss. In addition, changes in self-evaluation and self-perception can lead to carelessness and poor hygiene, making patients less likely to seek dental care (23–25). To date, no study has investigated the oral health of opioid dependent patients in comparison with healthy patients in a European country (25).

Therefore, the primary objective of this retrospective cohort study was to evaluate oral health and dental hygiene of patients with opioid addiction compared with a healthy control group. Specifically, the study examined several aspects including dental status, periodontal status, salivary parameters, dental care and dietary habits. In addition, the study sought to assess patients’ self-perceived pain and functional limitations. The null hypothesis was that these factors would not differ significantly between opioid-addicted patients (OAP) and non-opioid-addicted patients (non-OAP).

Despite the growing threat of opioid addiction and its significant health consequences, few studies have focused on the oral health of drug users, particularly opioid addicts (23, 24, 29). Existing research is often limited to specific populations, such as women or users of specific substitution drugs, and is always influenced by the particular characteristics of the site or region where it is conducted (23, 29, 30). In particular, there is a lack of evidence on the oral health of opioid-dependent patients in central European countries, especially in Germany. This retrospective cohort study fills a gap by examining the oral health, dental hygiene, dietary habits, self-perceived pain, and functional limitations in a cohort of opioid users. The results show differences between opioid-dependent and non-opioid-dependent patients in these categories, justifying rejection of the null hypothesis.

A major strength of this study is the inclusion of a matched non-OAP cohort, which allows objective comparisons to be made on all the parameters studied. Age and sex matching between the OAP and control groups increases the validity of the study in assessing oral health outcomes. However, some confounding variables, such as socio-economic background, were not fully accounted for. Significant differences were observed between the OAP and control groups in terms of education and place of residence. The lower level of education among OAP participants is consistent with German data showing that drug users typically have a lower level of education than the general population (31). In addition, the predominance of urban residence among OAP participants is consistent with Lenardson et al. (32), who reported a higher prevalence of opioid use in cities. However, contrasting reports from the USA suggest that non-medical opioid misuse also occurs in rural areas (33, 34). In this study, the urban residence of OAP participants probably reflects two factors: (i) easier access to drugs in cities in eastern Bavaria compared to rural areas, and (ii) the requirement that substitution treatment take place in an urban clinic (Department of Psychiatry and Psychotherapy, University of Regensburg), where the study cohort was recruited. Another potential confounding factor concerns the control group recruited from the Department of Conservative Dentistry and Periodontology. These individuals received regular high-quality oral health assessments, which may introduce selection bias. Control patients may inherently have better oral health and dental hygiene than the general population, potentially exaggerating differences between OAP and controls. The regular exposure of controls to professional dental care contrasts sharply with the likely limited engagement of OAP.

The main drug of abuse in the cohort of OAP was heroin, closely followed by cannabinoids and cocaine. Although opioid use was an inclusion criterion in this study, other studies have also reported heroin as the second or even the most commonly used drug (23, 35–37).

Gateway drugs used at an early age were sniffing substances, alcohol and cannabinoids. However, alcohol was also a frequent companion throughout the addiction phase and was used over a long period of time. In a Spanish cohort of drug users, half were chronic alcoholics, similar to the findings of this study (37).

The included cohort of OAP was recruited from substitution therapy and received appropriate medication and close medical care. In general, methadone is described as the most commonly used medication in substitution programs, followed by buprenorphine, levo-alpha-acetylmethadone (LAAM) and dihydrocodeine (35, 38–41). However, at the Centre for Addiction Medicine of the Department of Psychiatry and Psychotherapy at the University of Regensburg, patients were mainly treated with Polamidon (LAAM) or Subutex (buprenorphine). The reason for this is that the substitution regime is individually tailored to each patient in a “shared decision”-approach with the treating physicians. The main objective is to achieve safe and sustainable withdrawal from illicit opioid use. In order to optimize compliance and feasibility for patients, their individual previous experiences and wishes are integrated into the substitution plan, thereby also reducing side effects. If previous drug experience is still close, (D-) L-methadone is usually chosen, as many patients subjectively feel that it has a better shielding effect. L-methadone is usually preferred because of its lower rate of side effects. In the advanced stages of substitution treatment, buprenorphine is often used because patients report that they have better cognitive awareness and therefore fewer problems in everyday life or at work.

Significant differences were found in the oral health status of OAP and non-OAP. In terms of dental status, the median DMFT of opioid-dependent participants was significantly higher due to a large number of decayed and missing teeth. This means that the DMFT is even twice as high as that of the German cross-section of young adults (42). The DMFT of the control group, however, corresponds to the German average as determined in the German Oral Health Study V (42), which shows that the control cohort was representative and therefore effective. A similarly high DMFT, as described in this study, has also been reported in numerous other studies investigating opioid-addicted patients (23, 37, 43).

The assessment of periodontal health in this study revealed a significantly higher need for periodontal treatment in the test group. Status was assessed using the PSI, an index widely used by German dentists to define the periodontal treatment need. Existing evidence is often derived from the Community Periodontal Index of Treatment Need (CPITN), from which the PSI was derived, making both indices comparable (26). In general, other studies have also observed a high prevalence of periodontitis in opioid users (23, 44, 45). By contrast, Ma et al. (39) reported a lower rate of periodontal pockets, but only 10 index teeth were assessed and the number of missing teeth, possibly for periodontal reasons, was much higher.

Plaque accumulation on the tooth surface, an indicator of oral hygiene, was measured using the PI according to Silness and Loe. OAP were significantly more likely to have a PI categorized as ‘poor’ than participants in the non-OAP group. Similarly poor oral hygiene was reported for heroin users by Mehmood et al. (46). A possible explanation for the poor oral health can be found in the lifestyle of addicts. Studies have shown that opioid addicts have a preference for sweet flavors and meals high in sugar, both increasing risk of caries (47, 48).

Regarding the investigated saliva parameters of OAP, the literature presents controversial findings. While some authors, such as Protrka et al. (43), have reported a reduced flow rate of both unstimulated and stimulated saliva in drug users, suspecting that it could be due to smoking or taking medication or drugs that lead to xerostomia, the present study did not find significant differences in stimulated salivary flow rate or buffer capacity. Del Ribeiro et al. (49) recorded a mean salivary flow rate of 1.31 ml/min, which is within the physiological range and comparable to the flow rate observed in both OAP and non-OAP. The saliva has a bicarbonate buffer system that counteracts acidic cariogenic attacks. As saliva secretion increases, its bicarbonate content rises, resulting in a higher pH. This neutralizes the organic acids in plaque and reduces the risk of caries. Also, higher salivary flow rate yields a higher oral clearance, supporting increased pH-values (50). Conversely, low salivary flow is associated with limited buffering capacity. Consistent with the findings for OAP in this study, Mateos-Moreno et al. (37) measured a mean saliva pH of 6.8 in drug users, which was significantly different from the non-OAP group. The acidic salivary pH in OAP may be due to poor oral hygiene and high cariogenic activity. Setiawan et al. (51) showed that salivary pH increases after tooth brushing, suggesting that poor oral hygiene is a trigger for acidic salivary pH and high cariogenic activity.

The questionnaire aimed to analyze oral hygiene behavior and dietary habits, both of which may indirectly affect the oral health of drug users. It also included a self-assessment of dental health and quality of life.

OAP appeared to brush their teeth significantly less often, as several studies have shown (36, 45, 52). However, it was not just the frequency of brushing that was reduced, the quality was also inferior. The OAP in the present study showed significantly poorer interdental hygiene, with a lack of use of dental floss and interdental brushes, a finding confirmed by other studies (36, 52). In addition, awareness and knowledge of oral hygiene among drug users appeared to be limited. For example, 46% of drug users surveyed did not know whether their toothpaste contained fluoride, an important factor in caries prevention, compared to 28% in the control group (53).

Significant differences in dietary habits were found between the test and control groups. Although no increase in the consumption of traditional sugary foods like fruit, cakes, or pastries was observed, OAPs consumed significantly more sweets and sugary drinks—a trend consistently reported in numerous studies (36, 52, 54). This may be due to physiological and hormonal changes during drug withdrawal that affect food intake, nutrient absorption and the regulation of hormones linked to appetite and satiety (47). Furthermore, people addicted to opioids often replace protein and fat with sugary meals due to an increased preference for sweet tastes (55). Especially during substitution treatment, cravings for sugary foods, such as cakes and sweets, are often increased as a substitute for the drug (47). In addition, socio-economic factors, as indicated by lower levels of education in the test group, may also play a role by limiting access to a healthy, balanced diet. These findings highlight the need to promote a healthy, low-sugar diet for OAPs, not only to reduce the risk of sugar-related oral diseases such as dental caries, but also to improve overall health and well-being (56).

Drug users rated their dental status as significantly worse than patients in the control cohort, which largely corresponded to the objectifiable results of the dental examination. Although the OAP were well aware of their poor oral health and the limitations it imposed on their daily lives, they rarely visited a dentist. More interestingly, a clinical trial in Lithuania reported lower DMFT in OAP who visited the dentist at least once a year, highlighting the importance of prophylaxis and dental check-ups to maintain dental and periodontal health (30). Despite this, only a fraction of the OAP in this study underwent regular routine check-ups. 56% of OAP consulted the dentist only because of pain or other discomfort, which could have been avoided with regular prophylactic measures.

A reduction in the quality of life of opioid users was evident in many ways, including difficulties with eating, dry mouth and associated social problems, as shown by Mehmood et al. (46). For many, the poor condition of their teeth led to shame and embarrassment, resulting in reduced social interaction. The OAPs also reported a limitation to perform daily activities, however, no increase in absenteeism was found compared to the control group. This could be due to factors such as a heavy dependence on income from work, occupational structure or potential unemployment. However, the available data do not allow any definitive conclusions to be drawn.

A key strength of this retrospective cohort study is the inclusion of a matched control group, allowing objective comparison across multiple oral health dimensions. The comprehensive assessment of dental, periodontal and salivary parameters, together with self-reported behaviors and functional limitations, provides a holistic understanding of the oral health challenges faced by OAP. However, limitations include potential selection bias due to the recruitment of OAP exclusively from an urban substitution treatment center, which limits the generalizability of findings to rural or medically unsupervised populations. In addition, the retrospective nature of the study and reliance on self-reported data for certain measures may introduce recall bias or inaccuracies. Despite these limitations, the study fills an important gap in understanding the oral health of opioid users in Germany.

The clinical implications of this study are substantial and multifaceted. The findings highlight the urgent need for targeted oral health interventions for OAP. The high prevalence of dental decay, periodontal disease, and poor oral hygiene behaviors underscores the importance of integrating dental care into addiction treatment programs. Prioritizing routine dental check-ups, preventive care, and education on proper oral hygiene and dietary habits is essential to address these disparities. In addition, promoting caries prevention strategies, such as fluoride use and reduced sugar intake, should be integral to comprehensive care.

Opioid addiction is a growing problem in Germany, with a particular impact on oral health. Especially poor oral hygiene and dietary habits lead to common dental and periodontal diseases among opioid addicted patients, resulting in pain, functional limitations and tooth loss, which significantly affect their quality of life and social behavior. Social exclusion and feelings of shame, especially in public places such as a dental waiting room, are often underestimated factors affecting the dental care of drug users. Therefore, it is essential to establish easily accessible, confidential treatment services tailored to the needs of this vulnerable population. Good oral health is essential not only for the daily lives of those affected, but also for the rehabilitation process of drug users, as it has a profound impact on self-esteem and social integration.