I’m writing about Gary Gall’s thoughtful letter: “Cannabis is a safe, effective medicine” (June 10). Suppose that half, or even more than half, of the people who purchase medical marijuana don’t really need it for medical reasons. That means they are not buying their marijuana from drug dealers who get their marijuana via Mexican drug cartels. And it also means they are not buying their marijuana from drug dealers who also sell other, much more dangerous drugs, such as meth, heroin, and cocaine. Most marijuana consumers would prefer to buy locally grown marijuana of known quality, known purity, and known potency. But if they can’t, they buy black market marijuana.