viruses Review The Link between Cannabis Use, Immune System, and Viral Infections Sanjay B. Maggirwar 1, * and Jag H. Khalsa 1,2   Citation: Maggirwar, S.B.; Khalsa, J.H. The Link between Cannabis Use, Immune System, and Viral Infections. Viruses 2021, 13, 1099. https:// doi.org/10.3390/v13061099 Academic Editors: Maria Cecilia Garibaldi Marcondes and Marcus Kaul Received: 13 March 2021 Accepted: 3 June 2021 Published: 9 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Microbiology, Immunology and Tropical Medicine, School of Medicine and Health Sciences, The George Washington University, Washington, DC 20037, USA; jkhalsa@yahoo.com 2 Medical Consequences of Drug Abuse and Infections Branch, National Institute on Drug Abuse, National Institutes of Health, Bethesda, MD 20852, USA * Correspondence: smaggirwar@gwu.edu Abstract: Cannabis continues to be the most used drug in the world today. Research shows that cannabis use is associated with a wide range of adverse health consequences that may involve almost every physiological and biochemical system including respiratory/pulmonary complications such as chronic cough and emphysema, impairment of immune function, and increased risk of acquiring or transmitting viral infections such as HIV, HCV, and others. The review of published research shows that cannabis use may impair immune function in many instances and thereby exerts an impact on viral infections including human immune deficiency virus (HIV), hepatitis C infection (HCV), and human T-cell lymphotropic type I and II virus (HTLV-I/II). The need for more research is also highlighted in the areas of long-term effects of cannabis use on pulmonary/respiratory diseases, immune dysfunction and the risk of infection transmission, and the molecular/genetic basis of immune dysfunction in chronic cannabis users. Keywords: cannabis; marijuana; cannabinoids; immune system; infections; HIV; HCV; HTLV-I/II 1. Introduction Today, marijuana (cannabis) is the most frequently used drug in the world, with over 188 million users, or ~2.5% of the population that is 15–64 years of age [1]. In the United States, the percentage of people aged 12 or above who were past year marijuana users increased from 11.0 percent (or 25.8 million people) in 2002 to 17.5 percent (or 48.2 million people) in 2019 [2]. Approximately 2 to 3 million new users of marijuana are added each year with about 1.1% becoming clinically dependent on it [3]. According to a recent annual survey of high school students, known as Monitoring the Future (MTF), the total annual marijuana prevalence rose by a significant 1.3% to 23.9% in 2017 with prevalence of 10%, 26%, and 37% in 8th, 10th and 12th graders, respectively, whereas daily marijuana use remained at 1%, 3%, and 6%, respectively [4]. In addition to problematic cannabis use, an estimated 38 million people are living with human immune deficiency virus infection (HIV [5]), 170 million people with hepatitis C virus (HCV [6]) infection, 10–20 million people with human T-lymphotropic virus type 1 (HTLV-1 [7]), and an estimated 161 million people have been infected with coronoavirus, SARS-Cov2 [8], in the world. Cannabis use is associated with a wide range of adverse economic, social, psychosocial and health consequences. The psychosocial consequences of marijuana use—such as dropping out of school, poor school performance, and antisocial and other behaviors among youth—have been the subjects of many reviews/publications. The health consequences of cannabis use involve almost all physiological and biochemical systems including the immune, cardiopulmonary/respiratory, hepatic, renal, endocrine, reproductive, and central nervous systems, as well as genetics and general health [9–12]. This review presents current research on the impact of cannabis use on the immune system which, in turn, may lead to increased Viruses 2021, 13, 1099. https://doi.org/10.3390/v13061099 https://www.mdpi.com/journal/viruses