Before continuing, many marijuana users aren't serious cases, and usually smoke only once or twice a month. This doesn't affect them so much socially or mentally. It is much more like having a beer now and then. If you want someone to stop because of their health, do some serious research on sites that are not regulated by the government or that carry a religious affiliation. A site that is very good at being neutral and providing raw facts on all drugs is called Erowoid. Think of your own reasons for wanting your friend to quit. Is it because it's bad for them? Because it is illegal? because it makes them different?


Steps

1. Do not make them watch an anti-drug commercials. These often do not tell the truth about marijuana, making it sound like crack or heroin. In truth, the drug is not as potent; the only serious cases are with "potheads" (someone who gets high at least once a day, or by themselves often) in which case you can probably convince them to look at themself as an addict.
2. Do not play up the health risks if you smoke cigarettes yourself - sure, one joint may have 6 times as much tar as a cigarette, but most users have one joint a week, while most smokers have 20 cigarettes a day, and heavy smokers may have 60 or more.
3. Argue that they are not making things better being stoned all the time. It probably gets boring, and a break now and then is good. Things will only get more depressing deep down and worse on all fronts. After serious use of marijuana, there is usually a "down period" that can last a week or two, depending on the amount taken in. This often dissuades people from quitting.
4. Tell them how you feel about their drug use and how it really affects you. Tell them it's bad for them, and you don't want them to be controlled by the drug. Try to negotiate to get them to smoke less.
5. Don't write a letter, as this is bad form. Muster up some backbone and tell them to their face. This helps them see that you aren't just being a jerk (as they see it) but as looking out for them. Also, you can get more emotion in a face than you can in a letter.
6. Talk with them, and treat them like the real person that they are. If you begin acting like they are something the cat dragged in, or acting super polite it will most likely become very irritating and make the distance between the two of you bigger.
7. Help them to learn to relax and to have fun without drugs that isn't expensive. Think of activities you could do as friends in something that interests them that is free or doesn't cost much.
8. Understand that the addict must want to change for themselves - you must convince them to responsibility for the addiction and destructive behaviors.
9. Realize that when they try to stop that someone who used daily is probably going to feel confused or anxious. Stay with them as much as you can for a while so they don't get stressed or even bored, begin smoking and get addicted again.

Tips

• Don't be aggressive towards the smoker.
• Watch the film on Google Video "Grass: The War on Drugs" and learn the history of marijuana's illegality. There are also numerous other movies on this topic.
• Remember, it is your friend's decision to stop, not yours. Respect their decision.
• If they smoke once a day or more, challenge them to reduce their use of the drug to maybe once a week, once every two weeks, etc. If they rise to the challenge, there is a good chance they will be better off for it.
• Don't constantly nag a person--it won't help much. Do you like being constantly nagged to stop doing something, such as playing video games or watching movies?

Warnings

• Do not show them "Reefer Madness" because this movie is much too "old school" and comical to anyone in this day of age. Your adult parents or friends will most likely especially think so if they already smoke marijuana. There’s a really good chance they’ve already seen it in the past anyway, and probably as a joke for fun with their buddies being the pot smokers they are.
• Tests have shown that those who use marijuana heavily before they are adults (between 18-21) often have a permanent drop in IQ. For information, check out "Parents' Guide to Marijuana" by Dr. Mitch Earleywine.