Background: In recent years, the use of medical cannabis rapidly increased among cancer patients in Israel. Yet, cannabinoid receptors are abundantly expressed on immune cells and modulate their activity. It is abundantly being use by metastatic NSCLC patients, shortly following diagnosis and is taken in parallel to first line treatment with ICI. Recent studies suggested that the use of cannabis may reduce the efficacy of ICI. However, these studies were biased by the heterogeneity of patients and the increased use of cannabis specifically in highly symptomatic patients with high disease burden. Methods: We first tested the interaction anti-PD-1 antibody and Δ−9-tetrahydrocannabinol (THC) in a preclinical model consisting of CT26 tumor-bearing mice, and examined the effects on tumor size, T-cell infiltrates, and mice survival. Mice were euthanized when tumor volume reached above 700 mm³. Next, we conducted a retrospective study of NSCLC patients, treated at a tertiary center, and included all consecutive patients treated with a single agent pembrolizumab as a first line treatment for advanced disease and evaluated clinical outcome and response to treatment. Results: Studies using the CT26 mice cancer model, indicated a potential beneficial effect for the combination an anti-PD-1 antibody and THC with a median overall survival (OS) of the mice receiving no treatment, THC, anti-PD-1 antibody or their combination being 21 days, 24, 31 days and 54 days, respectively (p < 0.05). Data of 201 NSCLC cancer patients who received first-line single agent pembrolizumab for metastatic disease, 102 (50.7%) patients received cannabis and 99 (49.3%) were cannabis naïve was analyzed. Their median age was 68 for the cannabis treated group and 74 for the cannabis naïve (p = 0.003), 34 (34.3%) in the cannabis treated group and 62 (60.8%) for the cannabis naïve were women (p = 0.002). Similar distribution of histology, smoking status and PDL1 expression was noted between the groups. The efficacy of pembrolizumab, as determined by time to progression (TTP) was similar for cannabis-naïve and cannabis-treated patients (6.1 vs. 4.8 months, respectively, p = 0.386), while OS was higher, though not statistically significant, in the cannabis-naïve group (54 vs. 23.3 months, respectively p = 0.08). Conclusions: Both preclinical and clinical data suggest no deleterious effect of cannabis on the activity of pembrolizumab as first line monotherapy for advanced NSCLC. The differences in OS can be most likely by attributed to higher disease burden and more symptomatic disease in the cannabis-treated group. While additional validation is required, these data provide somewhat reassuring data regarding the absence of a deleterious effect of cannabis in this clinical setting.