The prevalence of COTC-related adverse events (3.2% among COTC users) is slightly lower than the 4.6% reported among patients prescribed complementary and alternative medicine in a European hospital setting  and the 6.5% reported in a U.S. hospital-based study of conventional allopathic drugs  but considerably higher than the 0.4% reported in a 5.5 year prospective study on patients prescribed alternative medicine [32, 44]. Methodological differences in study designs and study samples account for the wide range of adverse drug event rates. The results of this study, nonetheless, expose cases of COTC-related adverse events that had not been detected by previous clinic-based studies in Hong Kong. Since the majority of individuals with adverse events in our population-based study did not seek any type of professional medical treatment (and none sought emergency care), COTC-related adverse events reported primarily by emergency room clinicians appear to represent a small fraction of COTC-related adverse events in Hong Kong. Our results thereby indicate that COTC-related adverse events are an under-appreciated public health issue that may require greater scrutiny and improved surveillance. Since the list of adverse drug reactions includes many items which may be quite mild (such as minor skin irritation), the clinical significance of these adverse events is unknown and represents a limitation of the study. Nonetheless, approximately 1% of the COTC users (n = 8) had an adverse reaction for which they sought medical treatment, giving a rough estimate of more severe adverse events in the population. Although a previous study conducted on emergency room patients in Hong Kong reported that rashes and systemic allergic reactions comprised over 90% of the adverse events [35, 45], our study found that these conditions only represented about half of the adverse events in the general population while dizziness, nausea, vomiting, and gastro-intestinal problems comprised most of the remaining cases. Adverse events such as sleep disturbances, exacerbation of existing illnesses, and heart-related problems that were previously unreported in the COTC literature were also shown in this study.
Despite government efforts to promote safe COTC use through more stringent labeling requirements, over one third of respondents still find COTC labels to be unclear. Part of the reason for the ambiguity is that many of the listed drug actions, such as “dispelling dampness” or “normalizing the gall-bladder”, require advanced understanding of TCM concepts. In our study, the observed preference for unreliable sources of COTC information (e.g. mass media magazines) was shown to be the primary behavioral risk factor for COTC-related adverse events. Health information presented in the mass media is often inaccurate or incomplete [43–47]. Even among Hong Kong Chinese, a population with a long tradition of TCM use, TCM-related misconceptions and low risk perception of COTC harms are pervasive. Hence, strategies promoting safe COTC use must also include raising public awareness of alternative medicine safety. Our study corroborated findings from other regions that showed low risk perceptions related to complementary and alternative medicine use [25, 48].
The importance of increasing risk perception is particularly necessary since the inclination to self-manage health problems is strongly associated with alternative medicine use [49, 50]. In our study, COTC-users self-medicated without consulting TCM practitioners. Case reports about the adverse effects from misuse of complementary and alternative medicine from self-treatment abound in the medical literature [51–53]. It is particularly noteworthy that a large proportion of adverse event victims reported books as a common source of COTC information, even though none of those individuals were health professionals. Self-management of health using COTC among people untrained in TCM precepts has great potential for inappropriate COTC use, particularly since TCM users in Hong Kong possess lower educational levels . TCM treatments rely upon holistic diagnosis of the underlying syndrome while the prescribed treatments for a particular symptom may vary greatly between individuals.
In addition to addressing pervasive COTC misconceptions, there appears to be a need to reduce barriers to obtaining reliable drug safety information. Past surveys of Hong Kong pharmacists  and western-trained medical doctors  demonstrated a low level of TCM knowledge. Greater dialogue between TCM manufacturers, retailers, and Western health professions is required to develop effective safety measures for COTC users.
The trend towards greater alternative medicine use worldwide necessitates not only stringent labeling regulations and better consumer risk communication, but also improved surveillance of adverse events. The much higher rates of adverse events uncovered by this population-based study mirrors findings from other countries which found self-reported adverse events exceed those noted by doctors [57–59]. These findings indicate that adverse event reporting in an outpatient setting (e.g. web-based reporting or adverse event hotlines) should be explored. Improved surveillance of complementary medicines should be prioritized by governments in order to provide more comprehensive safety information for health professionals and consumers.
The main limitation of this study is the lack of clinical validation of self-reported adverse events some of which may have been unrelated to COTC use. Alternatively, it is also possible that some COTC-related adverse drug reactions may have not been recognized as such by users. Even among the valid cases of COTC adverse events that were captured by the study, it was not possible to determine whether poor drug quality, product misuse, or drug interaction was the underlying cause of the adverse event. Moreover, the direction of the positive association between adverse events and greater information seeking behaviors (e.g. reading labels) is unclear. However, recall biases are likely to be moderate due to short time frame of the recall period (past year). Lastly, the reliability of the summative scales (Cronbach’s α ranging from 0.52 to 0.68), indicated that these scales should only be used for exploratory purposes.