The results of this study showed that the majority of patients with lung cancer had a history of O&D use, and the use of O&D may be associated with an increased risk of LC. Also, in this study, there was a dose-response relation between O&D use variables and the risk of LC, and by increasing O&D use, the risk of this type of cancer increased as well. A study conducted previously in Tehran, Iran, showed that opium ever-use increased the odds of LC over seven times; and a higher frequency, longer duration and higher cumulative use (smoking or ingesting) of opium, showed a significantly increased risk of lung cancer. These researchers also found a significant (p<0.001) positive trend in the association between lung cancer and duration of opium smoking or ingesting (9).
However, the relation between O&D use and cancer may be confounded by other risk factors such as age, gender, cigarette smoking, and alcohol use (37). In Iran, opioid consumers are mainly older people who are more likely to be cigarette smokers as well (38). However, after adjusting for confounding factors, including cigarette smoking, alcohol use, education , and diet, the relation between LC and opioids remained significant in our study.
Meanwhile, opioids may also be used after the establishment of cancer and in order to relieve pain. Therefore, in this study, in order to prevent reverse causality, the history of O&D use from the past until two years before the diagnosis of cancer was investigated, although many previous studies did not included a lag time for exposure data collection (4, 9).
Many mechanisms have been proposed about the carcinogenicity of O&D. Studies have shown that O&D and its alkaloids, including morphine, have mutagenic effects (22). Empirical studies have shown that pyrolyzed opium has mutagenic effects on salmonella strains (39). Also, pyrolysates and morphine alkaloids have led to sister chromatid exchange in human lymphocytes and morphological changes in cultured Syrian hamster embryo cells (40). They have also caused carcinogenic changes after being injected under the skin, inside the trachea, or into the gastrointestinal system of rats (39). It has also been shown that aromatic hydrocarbons released from burning opioids, indirectly lead to DNA damage and, as a result, may stimulate mutagenic mechanisms (41). However, the carcinogenic mechanisms of opioids have not been thoroughly identified yet, and further studies are required.
It is worth mentioning that many chemicals are added to opioids during their processing, which may have carcinogenic effects, as well. One of these chemicals is lead, which is added by drug dealers to increase the weight of the product, and consequently increase their profit. In the studies conducted on opioids and addicted people in Iran, the amount of lead in the opioids and blood samples of addicts was much higher than usual and at levels that could cause severe health effects (42). Studies have investigated the relation between occupational exposure to lead and the risk of lung cancer; and have shown that exposure to lead increases the risk of lung cancer by about three-fold, and the relation is dose-related (43).
In our study, we used the median of variables in the control group as the cut-point, because the median is robust to outliers. In many previous studies the median was used as the cut-point as well (4, 19, 21). As shown in the results, the median of daily opium consumption among participants of the present study was 4.5 gram per day, which is higher than previous studies. For instance, Shakeri et al. investigated the association between opium consumption, and pancreatic cancer and reported that the median of daily opium consumption was 0.4 grams among participants in Tehran, Iran (37); and in another study which was done among oesophageal squamous cell carcinoma (ESCC) cases and controls, the median of opium consumption per day was 1.5 nokhod or 0.3 gram (4).
To the researchers' best knowledge, there is a limited number of studies that have investigated the effect of opium on LC. The first study dates back to 1977 and was done in Singapore with 233 cases and 300 controls and showed an OR of 2.4 (95% CI: 1.4 - 4.0)(8). After that, in 2012, a hospital-based case-control study with 242 cases and 484 matched controls conducted in Tehran, Iran, showed an adjusted OR of 3.1 (95% CI: 1.2 -8.1). These authors also found that the concomitant use of opium and heavy cigarette smoking, dramatically increased the risk of lung cancer by an OR: 35.0 (95% CI:11.4 - 107.9)(9). Another cohort study conducted in the northeast of Iran, showed that the opium use was associated with lung cancer, (OR 2.21, 95% CI: 1.44–3.39) in a dose dependent manner (ptrend<0.05). These authors also showed that opium users have a significantly higher risk of developing cancers in different body organs, which one of them is the respiratory system (44). Most of these studies confirm our results and show that opium and its derivatives can cause lung cancer.
Tobacco is the leading cause of LC in both men and women. In Iran, it is estimated that tobacco use accounts for more than 11000 annual deaths in all ages, and smoking has had an upward prevalence over the recent decades (45). Based on a survey that enrolled 5900 adults in Kerman, 8.3% of the study participants (15.5% of men and 0.8% of women) reported themselves as daily smokers (32). As expected in this study, a significant association was found between smoking and LC. However, the significance of the association was lost after adjusting for confounders, including opium. This finding might mean that in this population opium is a risk factor, stronger than cigarette smoking for lung cancer; and because our sample size was not large enough, cigarette smoking did not become significant in the adjusted model. Also it was interesting to see the odds ratio for lung cancer and opium was stronger in the non-smoker (OR=6.50) than the smoker (OR=3.27) population.
One study conducted in Golestan, Iran about opium use and mortality showed a significant interaction between opium and smoking in relation to mortality (46). However, in Mesjedi et al’s study about opium and lung cancer, due to zeros in the subgroups, evaluating the interaction of smoking cigarettes and smoking opium was not possible (9). Besides in pancreatic cancer patients the interactions between cigarette smoking (ever use) and opium consumption (ever use or cumulative dose) were not statistically significant (p=0.544 and 0.886, respectively)(47). Our study did not find a significant interaction between opium use and cigarette smoking either.
In this study, we aimed to investigate the prevalence of alcohol consumption among participants. As the majority of Iranian people are Muslim, we did expect the prevalence of alcohol consumption to be much lower in these people than other parts of the world. A previous pooled analysis showed a weak relation between beer consumption and lung cancer, and the OR for ≥20 g/day users vs nondrinkers was 1.42; 95% CI: 1.06, 1.90 (48). Alcohol consumption was included as a confounding variable in this study. However, we did not observe a significant relation, which may be explained by the low prevalence of alcohol users and non-existence of heavy alcohol drinkers in this study.
The dose response relation is one of the most important criteria for finding a causal relation between exposure, and outcome. In this study, participants with a cumulative use of ≤87.5 gram-years opium had an adjusted OR of 3.95, but participants with >87.5 gram-years had an adjusted OR of 4.79. The dose-response relation was also observed in a previous study about O&D and LC (9); and in a study on colorectal cancer patients, in which the OR was higher in high (OR = 7.7; 95% CI: 1.5 - 38.6) compared to low (OR = 3.4; 95% CI: 1.2 -9.2) opium users (20). Other studies have shown similar dose response relations between opium use and esophageal squamous cell carcinoma (4) and bladder cancer (18).
In this study, the minimum starting age of opium consumption in the control and case groups was 25 and 15 years, respectively. It is expected that people who start using opium at a younger age encounter the dangerous and mutagenic effects of this chemical more than others. In this study, we considered the never opium users as the reference group for comparison and found the adjusted odds ratio of 8.64 for people who started opium consumption sooner than others. Moreover, other studies have shown that the starting age of opium use might significantly affect the incidence of cancers such esophagus (4), and pancreatic cancers (37) as well.
Although in this study we used neighborhood controls, and we expected our participants to become matched for socio-economic status, but eventually we ended up with significant differences in educational status between cases and controls and we had to adjust for this variable. Similar to this, a study from India showed that as the level of education increased, the prevalence of opium use decreased (49).
One of the most critical challenges for any case-control study is to select the perfect control group. Most researchers agree, there is no ideal control group in a case-control study, and researchers need to think carefully about the representativeness of the control group, because a biased control group can lead to wrong results. In this study, we selected neighborhood controls because they were a better choice than hospital controls, as a variety of known and unknown diseases are caused by opium use. Also friend/family controls had the disadvantage of overmatching for opium use, as these habits prevail in specific social networks. Meanwhile, the interviewers who asked about exposure information were trained to explain for all participants that their information will remain confidential and reporting their opium use pattern correctly will not have any adverse consequences for them.