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Like you mentioned, Marijuana is not considered addictive when compared to other illegal drugs, but the substance still causes addictive tendencies. This is because other narcotics cause a physical addiction, while Marijuana's addiction is more psychological. When you experienced your first high, you felt a sensation that can only be brought on naturally during special occasions -- a Runner's High for example. You enjoyed the feeling of the high, so you smoked again. But I bet you weren't able to reach the level of pleasure you felt during your first high, and so you smoked more. Marijuana users smoke more and more each time they light up because they are chasing that first high; trying to reach that level again, but it's impossible. This is why Marijuana is considered a gateway drug. When users realize that they will never be able to feel that high again using Marijuana, they move on to other, more dangerous drugs such as cocaine. So in essence, you are not physically addicted, but your brain won't let you stop. It wants to feel that high again.
There are a few ways you can go about quitting Marijuana without seeking rehabilitation treatment. The first is to simply quit smoking cold turkey. I know that it sounds like it might give you an anxiety attack just thinking about it, but that's the most extreme consequence that could result from stopping. Unlike other drugs, Marijuana does not have strong physical repercussions when you give it up. There may be a few headaches, minor shaking, and some sleep loss, but this is to be expected when you are weaning your body off of any chronically used substance. In order to do this, you will need to change your lifestyle to fit your new habits. For example, find a friend that doesn't smoke, and who you can talk to when you have the urge to light up. Use them as a support system, and hang out with them often. If you surround yourself with people who do use Marijuana regularly, start to create a boundary with them. You don't have to bail on them, but if you know they're going somewhere to smoke, pass on going with them. Hang out with them when they're sober; when they are being who they really are. If your parents know about your issue, talk to them and let them know you're ready to give it up and ask them if they know of any ways to help. If they happen to smoke as well, let them know you're done with it and that you'd like them to respect your new values by not doing it around you. If you can't talk to your parents find a teacher, DARE Officer, or another adult that you can talk to who will be able to help you through the process. Telling someone does not mean that they will make you go to rehab. When you talk to them, let them know that you have a plan and that you would like their help in making sure you get all the way through it. Sometimes advice from an adult can help you see issues that you might not have thought of.
If you want to feel that high again, take up running, bicycling, or a similar physical activity. Scientists have made a connection between the high you get from smoking Marijuana to the feeling you get when you exercise. When you exercise for a period of time, your body's anandamide level increases and causes what is called a runners high, and is equivalent to that obtained by the THC in Marijuana. This is a much safer, healthier, and more positive decision for you to be making to feel "high." Not to mention, running can help get you into college, Marijuana cannot. You could do this as a hobby to help keep your mind off of smoking.
Also, please keep in mind that treatment is not necessarily a bad option. There are several types and options, and you might find one that fits you if you decide that you can't give up Marijuana cold turkey. You could find all of the options you have here: http://findtreatment.samhsa.gov/
I am glad that you recognize that Marijuana is not a good choice for you, and I would like to wish you the best of luck in beating the habit! Remember, there is always someone willing to help you when you need it! For more information, check out the rest of the Just Think Twice website. DARE to be strong, DARE to be yourself!