Editor's Note: This is the first installment of a special series examining the limitations of Mexico's most-trafficked illicit drugs: marijuana, cocaine, methamphetamine and black tar heroin. Mexico continues to be an important conduit for and producer of U.S.-bound illicit drugs. Much of our Mexico coverage tends to focus on Mexican criminal organizations and how their operations pertain to the United States. It is important to note that these criminal organizations, like licit entities, operate within geographic, political and economic constraints, and the commodities on which they profit likewise are constrained by a variety of factors. Mexican criminal organizations traffic four major illicit drugs: marijuana, cocaine, methamphetamine and black tar heroin. This series will examine the limitations involved in producing each drug and how those limitations affect the business interests of Mexican criminal organizations.