This study describes the behaviors and attitudes of cancer patients toward natural complementary therapies in four hospitals in Belgium. The most popular products reported were vitamins and herbs, with 86.4 % and 84.7%, respectively. Other products were reported by 69.5 % of the patients. Studies found that vitamins, herbal products, and minerals are the most popular CAM supplements used by cancer patients [3, 6, 18, 43]. This contrasts with our results, where other products were consumed more than minerals. As discussed above, there is no consensus on what CAM is, making comparing results between studies complex. These studies do not mention whether probiotics, gemmotherapy, birch sap, and omega 3/6 were included. Currently, studies on gemmotherapy and birch sap use in cancer patients are lacking. On the other hand, probiotics and omega 3 use have increased significantly [44]. This is in line with our results, where we have three times more patients taking probiotics (n = 33) and omega 3/6 (n = 35) than birch sap (n = 11) and gemmotherapy (n = 12). Moreover, they seem pretty promising as complementary therapies in some cancer treatments. In vitro and in vivo studies on colorectal cancer models have proven the efficacy of probiotics’ antiproliferative and apoptotic benefits [45, 46]. Furthermore, omega 3 appear to have anticancer colorectal activity [47].

It has been shown previously that socioeconomic status, geography, and religious and spiritual beliefs influence the use of CAM [3, 7, 18]. Molassiotis et al. [6] found that herbal supplements were the most used therapy among all European cancer patients, but homeopathy was the CAM most used in Belgium. Homeopathy and BFs were the least used NHPs in our study. The different number of CAM users may explain this difference. Indeed, Molassiotis et al. [6] interviewed a sample size of 18 CAM users representative of Belgium, whereas we counted 57, about three times more. On the other hand, Saudi cancer patients are more likely to consume Zamzam water and camel products, such as milk or urine. This highlights how geography, religious and spiritual beliefs can influence CAM consumption [7, 36]. Moreover, being a woman under 60, having a higher education level and having a breast cancer diagnosis are predictors of CAM use [4, 6, 43].

Our results also show that 52.5 % of the patients consumed at least five different supplements. This high level of consumption highlights the importance of educating patients and HCPs about CAM-drug interactions risk [19]. In addition, we found that 33.3 % of the patients increased their consumption. Interestingly, the potential for CAM-drug interaction with cancer therapy was significantly associated with the number of CAM supplements taken. Even though all these supplements are at risk of interactions, natural substances can be classified into two categories.

On the one hand, some substances, such as antioxidants and herbal products, have a wide range of potential interactions. This makes them more likely to interact with conventional cancer therapy. For example, laboratory results showed that vitamin C can reduce the effects of chemotherapy drugs such as anthracyclines, bleomycin, bortezomib, and cisplatin. This is also the case for vitamin E whit tamoxifen antagonistic effects [24, 27]. In their study, Fasinu and Rapp [26] identified six herbal products - echinacea, garlic, ginseng, grapefruit juice, milk thistle, and St John’s wort - that have shown clinically relevant interactions with specific chemotherapeutic agents. For example, a patient on imatinib for seven years developed hepatotoxicity symptoms only three months after consuming an energy drink containing ginseng. When the ginseng-energy drinks were stopped, liver dysfunction disappeared. In addition to ginseng, energy drinks contain various compounds that may have contributed to liver dysfunction. In contrast to all these substances cited above, homeopathy and most minerals are natural substances unlikely to interact [27]. Furthermore, screening interactions for CAM-drug interactions through software is not as simple as identifying drug-drug interactions. Indeed, some programs only provide information on drug interactions, while others do not encompass all products on the wide NHPs market. This incongruence was highlighted by Végh et al. [48] among three different interactions databases.

Our survey found that 83.1 % of the patients believed complementary therapies could benefit CM. Our results also reveal that patients did not perceive complementary therapies as a threat to public health (89.9 %). Although, many are unaware that these non-mainstream medicines could interfere with their conventional treatment [3]. Our study found that most patients experienced beneficial effects and only a few reported adverse reactions. Borm et al. [49] reported similar results. It is also important to note that most patients cannot distinguish between dietary supplements and drugs due to their similar shapes, packaging, and trade names. Visual differences can be so slight that it is almost impossible to tell them apart. However, as mentioned in the introduction, the requirements for obtaining drug status are significantly higher than those for marketing authorization dietary supplements containing NHPs. Moreover, the European directive (2002/46/CE) [12] permits the free circulation of NHPs as dietary supplements, allowing patients access to a wide range of NHPs. Healthcare professionals should, therefore, educate patients about these products and make them aware of their potential risks [16, 17, 50].

McFadden et al. [40] investigated the relationship between attitudes toward CAM and their use in 65 healthy graduate students. They identified three factors influencing the use of CAM: a) possession of “philosophical congruence” with CAM, which occurs when a patient identifies with (aspects of) the CAM modality’s cultures [51] b) dissatisfaction with CM and c) holistic balance. A philosophical orientation congruent with CAM therapies was significantly correlated with present use (r=0.41, p = 0.001). A systematic review [38] showed that positive attitudes toward CAM and dissatisfaction with CM were the main reasons for CAM use among the general and condition-specific population. Our study focused on the nine items of philosophical congruence with CAM. According to our results, eight items seem to be consistent with those of McFadden et al. [40]. The only contradictory result is that 73.9 % of the patients did not believe complementary medicine is more effective than CM. This difference may be explained by our study involving cancer patients who were more aware that complementary therapies cannot replace conventional treatment. This observation also follow an Italian multi-survey led by Berreta et al. [5], which showed that almost all patients interviewed trust CM and oncological treatments. Most patients underlined the importance of considering a patient’s body, spirit, and mind when treating them. They also emphasized the importance of treating the whole person and integrating health beliefs and values into the healthcare process. This highlights the need for integrative oncology in the future.

A cancer diagnosis appears to be an essential factor that can influence the consumption of CAM. Indeed, the use of these products tends to increase at the time of diagnosis [5]. Buckner et al. [3] showed that biological products such as green tea, curcumin and ginger were consumed by 15 % of the patients before the diagnosis, while their consumption jumped up by 52 % of the patients after the diagnosis (p < 0,01). A European study [6] showed that CAM use was lower before diagnosis and increased by at least 30% after their diagnosis and that herbal medicines’ use tripled. Surprisingly, our study found that NHPs use was mostly initiated before the diagnosis, except for other products (i.e., probiotics, birch sap, gemmotherapy and omega 3/6). We conclude that the diagnosis did not affect the initiation of the type NHPs’ use, as demonstrated by the the \(\chi ^\) test of independence. Our results match those of Horneber et al. [21], who did not find a significant difference between the current and past use of CAM. This result can also be due to selection bias because the questionnaire naturally attracted more patients who use NHPs. However, 72.7 % of the patients reported significant changes in their consumption since their cancer diagnosis. In fact, we found that 15.2 % of the patients had reduced their consumption. This behavior change might have been explained by a discussion with their HCPs, who advised them to reduce their consumption of these products due to a lack of information about them. In 2012, a systematic review [52] reported that the prevalence of any Traditional, Complementary, and Alternative Medicine (TCAM) in the general population was up to 76 %. Moreover, the reported prevalence seemed to be underestimated. We found that most patients used NHPs before diagnosis, possibly due to increased use in the general population. Regardless of the initiation of CAM use, we showed also that the frequency of use varies over time. Therefore, HCPs must regularly inquire about their patients’ use of NHPs to detect any potential interactions [22].

We found that the primary source of information varies depending on the type of complementary therapies. On the one hand, media, family, friends, and their close circle were the most common sources for using of BFs and homeopathy, EOs, herbs and other products. These results align with previous studies [5, 6, 18]. Huebner et al. [39] demonstrated that 46 % of the patients trusted naturopaths and non-medical practitioners regarding CAM products. In our study, we observed that the homeopath was the second most influential source of information after family, friends, and the close circle for the consumption of homeopathy and BFs. Interestingly, a study [34] found that patients would like to have more information mainly from their oncologist. They also reported that complementary medicine users trusted the received information (82%), no matter the source, but almost all (73 %) admitted that additional information would be necessary. On the other hand, HCPs were the main source of information about vitamins and minerals. This is not surprising since these supplements (vitamins and minerals) are often prescribed or advised by doctors. We must highlight that our study did not distinguish prescribed from non-prescribed supplements. Nevertheless, compared to other studies [5, 6, 18, 34], we categorized sources of information according to the type of NHPs.

According to our survey, the main reasons for using NHPs shared by patients were to boost their immune system and to reduce one or more side effects of their oncological treatment. These results are consistent with several studies [20, 39, 53]. Keene et al. [10] used a method of grouping to identify cancer patients’ motivations for consuming CAM. Based on this systematic review, the most common reasons people sought CAM were to influence their cancer and to treat symptoms or side effects of their cancer. The primary reason for including “influence general health” in the category was to increase immunity. During the COVID-19 health crisis, there was a significant interest in some NHPs due to their perceived “immune-boosting” effects. This led to a surge in the consumption of various products, including vitamins C and D, zinc, omega 3, and herbal products such as garlic, turmeric, and ginger. Consequently, the daily intake of vitamins and herbs significantly increased during this period [54, 55]. In an era when immunotherapy is increasingly becoming a promising cancer therapeutic approach, probiotics, and omega 3 may be crucial to their efficacy thanks to their influence on the microbiome. Indeed, multiple studies [45, 46, 56, 57] have shown that the type of microbiome in a patient could be the main factor in the immunotherapy response. Furthermore, mistletoe may also enhance immunotherapy effectiveness through its immunomodulatory properties [56]. Identifying the reasons for using CAM could help HCPs discuss its use with their patients. Several studies [30, 31, 58] noted that one of the most common reasons for the nondisclosure of CAM use was only doctors not asking about CAM intake.

According to our findings, 74.4 % of the patients did not want to postpone/avoid conventional therapy. This result is consistent with integrative medicine, which implies a more patient-centered approach. Using these therapies makes patients feel that they have more control and are actively involved in their treatment [3, 27, 49].

One of the main strengths of our study is the distinction between the various types of NHPs and how patients consume (i.e., the source of information, the initiation time, and the way they experience NHP consumption) these NHPs. Identifying the type of substances consumed daily would make it possible to target these substances for potential drug-CAM interactions. Our study incorporated certain substances such as gemmotherapy and birch sap. To date there have been no studies of patients’ habits toward this type of substance. Despite the small numbers of patients consuming these substances, more studies are needed to eliminate any possible risks.

Furthermore, the fact that other substances, such as probiotics and omega 3, are promising potential adjuvants confirms the importance of integrative oncology. Indeed, despite the many risks these natural substances pose, they can be beneficial in some way. Knowing that diagnosis influences patients’ drug-taking behavior highlights the importance of healthcare professionals discussing their patients’ drug-taking habits with them. Identifying patients’ beliefs about CAM use benefit conventional medicine by incorporating some aspects of CAM into traditional medical treatments.

Despite this, our analysis contains some areas for improvement, including a low number of questionnaires due to many incomplete questionnaires (n = 237). As such, our study only reports the use and experiences of a small French-speaking cancer patients’ sample in Belgium. The COVID-19 global pandemic may be responsible for fewer visits to oncologists and hospital admissions for cancer patients. Angelini et al.’s systematic review and meta-analysis [59] found a significant decline in visits worldwide from January to October 2020 compared to pre-pandemic times. In Europe, overall visits for cancer patients decreased by 39.0% (-46.7; -31.3). Additionally, we investigated the use of complementary therapies alongside conventional treatments. Therefore, our results are not representative of the oncological population in Belgium. Further studies are necessary to investigate the use of CAM, experiences, and beliefs of cancer patients in a much larger sample, alongside CM but also instead of CM. Using the Likert scale can be both a strength and a weakness. The Likert scale provides more details about perceptions, opinions, and behaviors than binary questions and is quicker to fill out for respondents than open-ended questions. However, filling out questionnaires can be long, and patients may lose patience and choose their answers less attentively over time. Respondents may also avoid extreme items to appear more “normal”, referring to response bias. Because the translation from English to French, some questions may be interpreted differently by respondents, especially items regarding patients’ attitudes toward complementary therapies. Finally, using a self-completed questionnaire compared to interviews may be the reason for our low response rate, as suggested by Horneber et al. [21]. Moreover, using interviews would allow us to determine the exact consumption of patients and establish an objective classification based on assigned status according to legislation. However, a recent systematic review [10] showed that the difference in the prevalence of CAM use between self-completed questionnaires and face-to-face or telephone interviews was not statistically significant.