Final answer:
The statement about marijuana being essentially harmless shows a lack of understanding regarding substance use disorders and the potential risks of using psychoactive substances, even if they are legal or commonly used.
Step-by-step explanation:
The client statement that indicates a knowledge deficit related to a substance use disorder is: "Marijuana is like smoking cigarettes. Everyone does it. It's essentially harmless." This statement reflects a misunderstanding about the potential risks and consequences of marijuana use. While it is true that the legal status of marijuana is in flux and that it may be used medically or recreationally in some areas, its use is not without risks.
Addiction, dependence, and negative health effects are possible outcomes of marijuana use, similar to other psychoactive substances. It is important to recognize that all drugs including legal substances like alcohol and nicotine, as well as illegal substances like opioids and hallucinogens, can lead to substance use disorders and require rehabilitation efforts.