I will only say that before you start any conversations with your child about drugs, make sure that you yourself have enough information on this topic, and do not retell to your child what you and him were told at school and on TV. Perhaps, by filling your own gap in this area, you will formulate your question a little differently.
In addition to the above, I would tell children that drugs often bring suffering than "high". The human body has ways of rewarding good behavior - sex, food, environment, communication with people, achievement of goals, love, cognition - all these things "rod" are much stronger, more pleasant and useless than exogenous psychotropic drugs, and they just seriously break natural mechanisms of reward. And if you do not do what the body rewards you with "internal" drugs for - you are unlikely to experience worthy pleasure from drugs, on the contrary, drug addiction lowers self-esteem due to public censure and decreased body functions + psychosis, abstinence, poverty, unsanitary conditions, not the most worthy social circle - all this makes drug addicts people who are psychologically completely different from the Indian gurus who have attained enlightenment, but simply sick people and criminals, both in a mental, physical and legal sense.
I would venture to express a seditious and radical thought - to tell the truth, instead of frightening. My son is 13 years old, the question is now more than relevant and I thought and read a lot on this topic. Below I will briefly outline what I came to.
The truth must always be told and everything, including the truth that he will definitely not die from marijuana / cannabis and most likely will not become addicted, that dependence on it occurs less frequently than dependence on alcohol, nicotine and nasal drops. That even opiates do not always become addicted, but if it does, there are already few chances to "get off". That in order to "get hooked" on opiates, a syringe is not required - opium and many of its analogues can be smoked in most cases. How and why a roof can fly away from hallucinogens, what are the benefits and what are the harm. To tell and explain everything, if you don't have such experience, find someone from your friends who is in the subject and ask him to participate in a conversation with the child.
The logic is simple, the worst thing in this situation for a parent is even not the drugs themselves, this is the protest age among adolescents. For many, especially boys, it does not end before the age of thirty. After all, we cannot always be near, we can only lay the correct understanding. Right is true. When a child begins to deal with drugs in life (and he will inevitably start, there is no need to harbor illusions on this topic) the worst thing that can happen is a teenage protest. Imagine that your son has found a normal, decent job, with funny, sociable, open people from whom you want to take an example. What happens if he finds out (and he finds out) that one of them constantly smokes? Someone sniffs. How long have you been sniffing? Five years already. And not dead? As you can see.
Teenage maximalism is turned on in full. "They lied to me. Parents are stupid, they don't know anything, I'll decide everything myself." I myself was such a teenager, I am not ashamed to admit it. And further from this thought to the desire to try everything in general - one step.
Therefore, even about the most terrible drugs - only the truth. Even about opiates. I explained through examples with my grandmother and a close friend. Grandma has cancer and it obviously hurts a lot. While she was in the hospital and after being discharged, she was given very serious analgesics, more potent than morphine. Why didn’t my grandmother become a drug addict and didn’t “hang around”, she didn’t take one dose, she “sat” on serious opioids for almost a month. Because grandmother has something to do in life. She has a family, hobbies, she has no need to run away from reality and "drown in grief." And about the other side of opiates, I asked a close friend who, the only one I know, ten years ago successfully "got off" from heroin and is now alive after meeting this habit. Then he worked in a rehabilitation center, helped people cope with addiction, and responded to my request with great enthusiasm. Usually, survivors share such things easily and often seek to help others. If you do not have such friends, call the hotline or one of therehabilitation centers, I'm sure some volunteer will volunteer for this conversation.
If a child during such a conversation looks extremely bored and sometimes giggles, you should know that you are doing everything right. There is nothing taboo in this topic, discussing it with parents and children is normal and even a little boring. More medical facts. Less pathos. Unforbidden, boring, and even poisonous fruit is not so sweet, is it?
Well, it depends on which side you look at. I believe that you need to tell about both, only for the child in simple words. I don’t know, of course, the age of your child and his degree of knowledge, but for arguments you can show terrible shots of an overdose (there are a lot of them on the Internet), embellish and say that it destroys lives, etc. In principle, I don’t need to list everything here, I think you know that too!
But it should be taken into account (even at a young age) that drugs are "good", but only for medical purposes.
If I had a child, that's exactly what I would explain. Since childhood, I would try to explain things from several points of view.