The title of this submission: " Marijuana Use <i>May Lead to</i> Testicular Cancer"<p>The title of the linked article: "Marijuana use <i>may increase risk</i> of testicular cancer"<p>Two completely different things. Moral: Don't invent headlines, copy them.<p>The study has no control group, so it cant't guard against confounding cofactors, such as genetic, environmental and lifestyle choices that lead to (a) an elevated risk of testicular cancer, and (b) marijuana use.<p>Suppose there is an unevaluated cofactor, like urban living or socioeconomic status, that increases the risk of testicular cancer and marijuana use. Such a cofactor would be easy to eliminate, at least on paper -- set up a controlled, scientific study in which two groups of subjects are required to smoke marijuana, or required not to.<p>That would be the only reliable way to eliminate cofactors and turn a correlation into a cause-effect relationship, which is why it's the gold standard for scientific studies. But in this case, it is clearly unethical -- imagine telling your subjects that they have to use an illegal recreational drug.