Health care systems are not only responsible for promoting the health of individuals in society, but also for protecting them from the financial costs of adverse health outcomes by reducing the burden of out-of-pocket payments for health care. Subsidy programs and the expansion of prepayment programs are at the center of these efforts [1]. Out-of-pocket payments are considered the worst and most inequitable form of healthcare financing [2]. From both a risk protection and equity perspective, this financing method has many adverse effects and places individuals at the highest risk of financial distress [2]. One of the phenomena that has exacerbated the problems related to this financing method in recent years is informal payments [3, 4]. Economically, informal payments are considered a form of direct out-of-pocket financing, as they have similar effects on demand and financial burden [5]. Informal payments are a major source of health care financing in many developing countries and a significant obstacle to health system reform [6].

Informal payments include illegal payments made to healthcare providers in exchange for services that should be provided free of charge to patients [7, 8]. These payments can take many forms, including cash payments, gifts, or goods and services [4, 9]. Under-the-table payments, black money, gray money, and corrupt payments are other terms used for informal payments [2]. Despite the high relevance of this issue, there is a lack of comprehensive, up-to-date data on the magnitude of informal payments in Iran, particularly in Tehran, which limits the ability to evaluate the effectiveness of policy interventions.

Informal payments for healthcare can be seen as a strategy to cope with resource shortages and poor performance on both the supply and demand sides [5]. On the demand side, patients may make informal payments in order to expedite their appointments, receive better and higher-quality services, gain easier access to care, obtain more attention, or establish a relationship with physicians or other healthcare workers to ensure prompt response in case future services are needed. On the supply side, providers may seek informal payments to compensate for low wages, meet financial needs, or achieve the desired income [5, 10]. However, informal payments are primarily driven by issues on the supply side, including low budgets for healthcare organizations and low salaries for staff, so that informal payments serve as a means of collecting additional funds for healthcare organizations in general, and for physicians, in particular [9].

These payments can have numerous negative effects, including reduced health equity, decreased access to and utilization of healthcare services, disillusionment in society, particularly among the impoverished, reduced motivation among service providers to deliver high-quality services, loss of confidence and a sense of servitude among healthcare workers, corruption within the healthcare sector, provision of inaccurate information regarding disease costs, and an increase in the patient’s share of these costs [11, 12]. In general, informal payments understate the true costs of the health system. This misinformation can distort health system decisions and policies and hinder health care system reforms [11, 13].

Numerous studies in developing countries and in various contexts, including Central and Eastern European countries and Central Asian countries, show a growing trend of informal payments [13, 14]. The prevalence of informal payments varies across countries, ranging from 2% in Peru to 96% in Pakistan [7]. Studies have reported the extent of informal payments in Bulgaria, Poland, Turkmenistan, and Tajikistan as (43%), (46%), (50%), and (70%), respectively. So that patients were forced to make informal payments for services that were legally free [15]. In Iran, in a study in Urmia, this rate was estimated to be around 30% [16].

Ensuring equitable financing of health care and protecting individuals from the burden of health care costs is a critical task for health care systems around the world, and health systems have adopted a variety of strategies and programs to achieve this. In Iran, the Health System Transformation Plan has been mandatory since May 5, 2014, to reduce out-of-pocket payments for patients hospitalized in hospitals affiliated with the Ministry of Health and Medical Education. The main goal of this plan is to eliminate informal payments and increase access to health services for the community. To this end, increasing physicians' income and improving patient access, especially in deprived areas, are proposed as the main strategies in this plan.

To address the scientific gap, this study is distinctive in several ways. First, it is conducted in the post-Health System Transformation period, which allows assessment of informal payments after policy interventions aimed at reducing out-of-pocket costs. Second, it focuses specifically on Tehran, where the healthcare system is highly centralized, and there is a high concentration of specialist and subspecialist physicians, providing a unique context compared to other regions of Iran. Third, this study not only quantifies the prevalence of informal payments but also identifies their determinants, providing evidence directly relevant for policy planning and reform. By emphasizing the Tehran context, post-reform timeframe, and policy implications, this study fills a critical knowledge gap and supports targeted, evidence-based interventions.

This study seeks to provide a precise estimation of the scope and magnitude of informal payments in selected hospitals in Tehran. The results are expected to fill the existing knowledge gap, support evidence-based interventions, and inform future reforms in payment mechanisms within the Iranian healthcare system. These findings can help policymakers make evidence-based policymaking to increase equity, prevent rising healthcare costs, and reform payment mechanisms in the healthcare system.

According to the findings, 19.94% of patients (67 out of 336) reported making informal payments. This percentage is higher than the figure reported by Zanganeh Baygi et al. (2018), who found that only 2% of patients made informal payments in hospitals in Zahedan [18]. However, this rate was underreported in the study by Yousefi et al. (2020), which found that 34% of hospitalized patients made informal payments [19]. In the study by Ghiasipour et al. (2011), this rate was also reported as 21% [20]. Or similarly, Hajian Dashtaki et al. (2018) found in their study that 27.2% of patients made informal payments [21]. According to the study by Parsa et al. (2017), a relatively high prevalence of informal payments was reported, at 63.8% [22]. The high prevalence of informal payments demonstrated in this study warrants attention and targeted interventions by policymakers. There is considerable evidence that informal payments exacerbate the financial burden of health care, particularly for the economically disadvantaged, and have negative impacts on equity and health outcomes [9]. Regardless of the motivations behind informal payments, they have widespread and adverse consequences for the health system. These consequences include increased health care costs, inequities in access to essential services, and ultimately inequities in health outcomes. Therefore, the most important consequence of this phenomenon is reduced access to health care services due to increased costs of services, which in turn exacerbates inequalities in health outcomes. As a result, individuals from low socioeconomic groups are disproportionately affected and face greater health challenges. Furthermore, informal payments reduce social justice and undermine public trust in the healthcare system and the government as a guardian of health [23].

Based on the findings of our study, among the 67 patients who made informal payments, a larger number (33 patients) made such payments to physicians, mostly in cash. This is consistent with the findings of Yousefi et al. (2020), which similarly reported that most informal payments were made to physicians; among these, general surgeons and urologists received the highest amounts, and these informal payments were often mandatory [19]. In a study by Parsa et al. (2017), it was shown that 63.8% of physicians who had the opportunity to receive informal payments did so at least once [22]. Numerous similar findings were observed in various contexts. For example, Liaropoulos et al. (2008) found that 36% of patients in Greece had made informal payments to physicians at least once [14]. Ozgen et al. (2010) also reported in their study that 75% of cash payments made to physicians in hospitals were made as informal payments [9]. In addition, Tatar et al. (2007) reported that informal payments were received more by physicians and surgeons [24].

Our study found that of the 67 patients who made informal payments, 31 made such payments to nurses, mostly in cash. Similarly, a study by Yousefi et al. (2020) also showed that after physicians, nurses received the highest amount of informal payments [ 19 ]. Liaropoulos et al.’s (2008) study found that 11% of patients in public hospitals made informal payments to nurses [ 14 ]. In contrast, this rate was reported to be only 5% in the study by Khodamoradi et al. (2015) [ 25 ]. Or in the study by Zanganeh Baighi et al. (2019), the average amount of informal payments to nurses was estimated at US$64 [ 18 ]. These findings reflect the profound impact of systemic and socioeconomic factors on this phenomenon. Evidence shows that informal payments are often seen in healthcare systems when financial support for health service providers is reduced due to adverse economic conditions, leading service providers to seek alternative sources of income. Indeed, in the informal market that is thus created in the healthcare sector, as in other informal sectors, informal payments to physicians, nurses, and other members of the healthcare team often remain unmonitored, illegal, and unreported.

The study findings also showed that of the 67 patients who made informal payments, 22 made these payments to other healthcare workers, mostly in the form of gifts. This is consistent with the findings of Liaropoulos' (2008) study, which showed that 8% of patients in public hospitals made informal payments to other staff [14]. In the study of Ghiasipour et al. (2011), it was shown that all 63 patients who made informal payments made these payments to hospital staff, and one patient made informal payments to a security guard [20]. In the study of Ozgen et al. (2010), this rate was reported to be 9% [9]. And in the study of Khodamoradi et al. (2015), the rate of informal payments to other hospital staff was estimated to be 17% [25]. These findings indicate that informal payments in the health system are not limited to physicians and nurses, but also include other personnel. As mentioned, one of the factors influencing the prevalence of such payments is the unfavorable economic situation and relatively low wages among healthcare workers, which have been pointed out in numerous studies [20]. Inadequate compensation for services provided, lack of regular payment of salaries, and inadequate government support measures have led some healthcare providers to turn to informal payments as a necessary source of income to fill financial gaps. The prevalence of this phenomenon among healthcare workers is a warning about the need to address issues related to healthcare worker salaries, which can help create a more transparent and equitable healthcare environment [25].

Our study findings showed that there is a significant relationship between the amount of informal payments to physicians and the length of hospital stay, which is consistent with the results of Ghiasipour et al. (2009) [20]. Evidence suggests that the severity of the patient's condition and the perceived need for specialized care or higher quality services are factors that influence patients' acceptance of informal payment requests. According to the results of the study by Vian et al. (2006), supporting quality improvement programs can be an effective policy to reduce informal payments. The findings of this study showed that patients are more likely to make informal payments when they expect to receive better and more timely healthcare services in this way [26]. Therefore, targeted interventions and strategies to increase the quality of healthcare and improve access to public services can help reduce the prevalence of informal payments.

This study has several limitations. First, data were collected through self-report questionnaires, so participants are subject to recall bias or underreporting, especially on a sensitive topic such as informal payments. Second, this study was conducted in a limited number of public hospitals in Tehran, and the results may not be generalizable to all hospitals in Iran. Third, the study was conducted cross-sectionally, which does not allow for the establishment of causal relationships between the variables under study. Finally, cultural and social factors influencing informal payment practices were not examined in depth, which may limit a comprehensive understanding of this phenomenon.

According to the findings of this study, about 20 percent of patients have made informal payments to healthcare workers. Given the negative effects of informal payments on equity and governance, it is essential that policymakers pay special attention to this phenomenon and adopt strategies to reduce its prevalence. While currently, limited efforts have been made to control informal payments, it is recommended that future strategies include regulating salaries, benefits, and medical tariffs, creating enforceable laws and regulations to combat informal payments, emphasizing programs to improve service quality, motivating healthcare workers, promoting accountability in the healthcare system, and changing public perception toward greater transparency and fairness in the healthcare system.