Inflammation is considered a process characterized by the activation of immune and non-immune cells. A number of mental disorders affecting the nervous system activate degenerative processes and inflammatory mechanisms. Substance use induces a cascade of inflammatory responses, including interleukins (ILs), chemokines, interferons (IFNs), tumor necrosis factors (TNF), and lymphokines. It was reported that substance use disrupted cognitive control through inflammatory processes, and this contributed to problematic behaviors, including increased substance use.²³ Neuroimaging studies reported evidence associated with METH-induced neuroinflammation. METH triggers undesirable levels of dopamine and glutamate release. Excess dopamine can cause oxidative stress and mitochondrial dysfunction at cellular level. In addition, excessive prefrontal glutamate release is associated with an increase in the expression of cellular pro-inflammatory cytokines.^24,25 Magnetic resonance spectroscopy studies with METH user reported decreases in the creatine plus phosphocreatine (Cr + PCr)/choline-containing compound (Cho) ratio and N-acetyl aspartate (NAA) concentrations, favoring neurotoxicity.²⁶ Nevertheless, these markers are neither practical, nor cost-effective. Although the central state does not directly imply the peripheral state, it may reflect it to a certain extent. This is because it was shown that neuroinflammation-induced cognitive impairment was associated with peripheral markers.²⁷ For the purposes of the present study, peripheral indicators were considered more easy-to-measure and more practical. As a matter of fact, whole blood count is the most widely used laboratory measurement today. And recently, new and more specific parameter were suggested for the detection of systemic inflammation. PLR, NLR, MLR, BLR, SIRI, and SII are among the most important of aforementioned parameters.²⁸ Despite, it originally found wide use in cancer research, it was gradually extended to other diseases involving inflammation. Accordingly, these markers were studied in schizophrenia and bipolar patients, where inflammation was known to have involved in etiopathogenesis in psychiatry. Higher SII, SIRI, neutrophils, and monocyte values were found in patients with schizophrenia and bipolar disorder, which were characterized by high inflammation burden, compared to healthy individuals. Previous studies suggested certain measures, which could be used as a reference during the identification and differential diagnosis of schizophrenia and bipolar disorder, indicating the biomarker potential thereof.²⁹ In the present study, white blood cell, platelet, basophil, monocyte, neutrophil, lymphocyte, SII, SIRI, and BLR values were significantly higher in patients diagnosed with SUD compared to healthy controls. A 2021 study investigated peripheral immune parameters in patients diagnosed with SUD and did not report any significant difference in the patients’ leukocyte and platelet counts, and concluded that the study results were associated with the limited number of studies on peripheral inflammation markers in the scope of substance use disorders in the relevant literature.¹⁵ METH is associated with permanent changes in neuroplasticity due to the disruption of the blood-brain barrier. It was suggested that this was induced by modifications in the immune system cells themselves and the signaling pathways used thereby. Therefore, METH is associated with inflammation.^30,31

Albeit the difference between the patient and control groups in the present study by MPV values, this difference was not significant. Nevertheless, as the MPV value increased, the SIRI value increased significantly. Previous studies, which compared the MPV value in schizophrenia patients with healthy controls, reported higher MPV values in the patients.^32-34 Another study with patients with schizophrenia and bipolar disorder, who used antipsychotics, reported that SII values were significantly higher compared to healthy controls. The same study suggested that the antipsychotic drugs use in the patients may have accounted for lower MPV values in schizophrenia patients.³² MPV value is a reflection of inflammation, and accordingly, previous studies reported that it increased in low-grade inflammations and began to decrease in high-level inflammations. In addition, it is affected by age, sex, body mass index, and antipsychotic drug use. Because the patients in this study were newly diagnosed and had not received any prior treatment and given that the patient group was matched with healthy controls by age and sex, the MPV value and its association with SIRI could indicate early signs of high inflammation in these patients.

Furthermore, the majority of patients used marijuana and synthetic cannabinoids. The oxidative load associated with additional substance abuse may prevent specific interpretation of the study results, therefore, certain properties of cannabinoids are reviewed below. These substances use the peripheral receptor CB2r (cannabinoid 2 receptor) of the endocannabinoid system and have an important role in immune modulation.³⁵ A study reported that the pro-inflammatory chemokine CCL11 (eotaxin-1) level was higher in patients with current cannabis use compared to healthy controls, yet there was no difference between addicts, who had not used cannabis for 2 months or longer, and healthy controls.³⁶ Certain previous studies found no significant differences by inflammatory marker levels, including IL-1β, IL-6, and TNF-α, between cannabis users and healthy controls.³⁷ Furthermore, current marijuana users had lower CRP levels compared to those never used marijuana for a 1-year period, based on the National Health and Nutrition Examination Survey (NHANES), one of the studies which used C-reactive protein (CRP) to investigate the association between marijuana use and systemic inflammation. Therefore, previous studies suggested that cannabis had an anti-inflammatory effect.³⁸

Similarly, another study reported lower biomarkers of systemic inflammation, including CRP, IL-6, and fibrinogen, in patients, who reported marijuana use in the past 30 days.³⁹ Continued use of cannabis within the last 30 days may provide a stronger anti-inflammatory effect compared to not using cannabis for 1 year (compared to older cannabis use).⁴⁰ All the above reports cast a shadow over the inflammatory burden of cannabis. In the respect thereof, when compared to other reports, there is far precise, clear, and numerous data indicative of the fact that METH is associated with inflammation. In addition, another point of consideration was that the patients included the study sample primarily used METH and consumed cannabis less frequently and in lower amounts. However, it is important to recognize that multiple substance use may interact with each other, potentially influencing the parameters we analyzed. In addition, the fact that METH-only users had significantly higher percentages of NLR, MLR, BLR, ELR, and neutrophil values compared to patients using METH + cannabinoid was consistent with above suggestion that METH was associated with inflammation. There was no significant difference by PLR value. Upon a review of previous studies with these parameters, a meta-analysis study in 2018 reported NLR, PLR, and MLR values as indicators of inflammatory activation in mood disorders.⁴¹ Whereas, schizophrenia studies reported that the NLR value was higher.^42,43 In a study conducted on heroin addicts, NLR and PLR were higher in heroin addicts compared to controls, and it was shown that these values were associated with the duration of substance use.⁴⁴ Another study compared patients with cannabinoid use disorder and opioid use disorder with healthy controls, and reported that there was no difference by NLR, where there was significantly lower PLR in the opioid use disorder group, and significantly higher MLR in the cannabinoid use disorder group compared to the opioid use disorder group.⁴⁵ Another study reported that NLR was significantly higher in individuals using synthetic cannabinoids compared to the control group, yet there was no significant difference was found in terms of PLR.⁴⁶ Nevertheless, in most of these studies, there is an issue associated with the limitation of confounding factors, and there were varied drug dose amounts and durations in the patients. In our study, almost half of the patients used METH regularly for nearly 5 years. In addition, there was a significant positive correlation between the duration of METH use and MPV. As is well known, platelets increase in size when activated and release inflammatory factors such as cytokines, chemokines, and coagulation factors. Increased MPV values indicate increased platelet function and are associated with inflammation.⁴⁷ Both acute and chronic METH use is associated with inflammation, and it is even mentioned that chronic use impairs both B and T cell functions of the immune system, making it vulnerable to opportunistic pathogens.⁴⁸ Moreover, indicators of high inflammation have been found in METH users even 12 months after discontinuation of METH.⁴⁹ There is more evidence pointing to an anti-inflammatory effect of cannabis use, even in chronic use.^50-53 A cross-sectional study involving long-term cannabis users found higher levels of circulating CRP and correlated this with 18 kDa translocator protein (TSPO) levels, emphasizing neuroinflammation.⁵¹ Ongoing cannabis use (e.g., last 30 days) may provide a more potent anti-inflammatory effect than more distant use (e.g., last 12 months).⁴⁰ Nevertheless, the effects of chronic and long-term use of both substances on inflammation vary from person to person and often interact with other factors.

Cigarette smoking increases oxidative stress and systemic inflammation in general, which may lead to changes in SIRI levels. Smoking may also indirectly affect SIRI levels as it may contribute to the development of various chronic diseases. According to studies, NLR and MPV/PLT ratios are higher whereas PLR is lower in smokers compared to the general population.^54,55 In smokers, high NLR and ELR and low LMR (lymphocyte/monocyte ratio) are associated with smoking.⁵⁶ On another note, a study by Lin et al⁵⁷ found that MLR was associated with smoking. A 2024 review reported that passive smoking was both associated with depressive symptoms and increased risk of inflammation. Another important finding of the same examination was that high blood cotinine levels (a nicotine metabolite) significantly increased SII and SIRI levels.⁵⁸ In our study, the majority of the sample were smokers, and smoking rates were significantly higher in patients compared to the control group. Although we excluded additional medical conditions that indicate a high inflammatory burden in the sample, we should mention smoking as a limitation.

The strength of the study was that the sample group did not have any additional psychiatric disorder. Patients with additional mental disorders that met the diagnostic criteria were excluded from the study, contributing to selective formation of the study sample. It is well established that SUD is accompanied by comorbid mental disorders, including depression and alcoholism disorder, to a high degree. Excluding this important confusing factor contributed to the relevance of the study data. Another strength of the study is that METH only use as well as the combined use cannabinoids and synthetic cannabinoids were considered in the sample selection. Other substances that were considered to facilitate inflammation, including opiates, hallucinogens, and sedative hypnotics, were not included. In addition, the patients in the sample were not previously administered psychotropic medication. This is because of the fact that psychotropic treatment might change immune function by altering the inflammatory and anti-inflammatory cytokine levels.^59,60

The limitations of the present study included the fact that poly-substance use could not be limited and parameters such as body mass index and smoking were not included. In addition, since alcohol use often accompanies substance use, this study ensured to only include patients who have not consumed alcohol in the last 6 months.⁶¹ However, another limitation of our study is that individuals who have not used alcohol in the last 6 months were identified through self-report. Additionally, since it was a cross-sectional study, the study results cannot be generalized outside the sample.

To the best information of the authors, this is the first study to report SIRI and SII values for SUD, i.e., the systemic inflammation markers in whole blood biochemistry. White blood cell, neutrophil, lymphocyte, BLR, SII, and SIRI values were significantly higher in MCUD patients. The NLR, MLR, BLR, ELR, and neutrophil percentage were significantly higher in patients with methamphetamine use only compared to patients using METH + cannabinoids and synthetic cannabinoids. These results indicate that methamphetamine and cannabis exposure affect blood parameters by elevating inflammatory markers through various mechanisms.

Inflammatory cytokines and inflammation cascades maintain their place in the etiopathogenesis of mental disorders with their effects on the central nervous system. The present study provides important data for further research on immune system mechanisms in substance use disorders, including SUD, and to develop convenient methods and markers that can detect these disorders to a certain extent.

BLR, PLR, NLR, MLR, SIRI, and SII are simple, noninvasive, inexpensive, and reproducible laboratory parameters that indicate systemic inflammation. There is a need for easily measurable biomarkers in mental disorders. We believe that furthermore comprehensive studies in this field, especially those that can establish a causal relationship, will strengthen the body of evidence.