The damages done by marijuana are done the same way as heroin, cocaine, and ecstasy.

Now that I have your attention, I'm going to explain why.

Heroin - Endorphine antagonist. Purges endorphines, the body's "good feeling" neurotransmitter. Endorphines quickly run empty, the body feels withdrawal symptoms of pain.

Cocaine - Dopamine reuptake inhibitor, as well as norepinephrine and seratonin in small amounts. Uses up all dopamine, the body's "reward" neurotransmitter. Dopamine neurons get frayed, the body feels withdrawal symptoms of punishment.

Ecstasy - Seratonin reuptake inhibitor, as well as dopamine and norepinephrine in small amounts. Uses up all seratonin, the body's "happiness" neurotransmitter. Seratonin neruons get frayed, the body feels withdrawal symptoms of sadness.

Marijuana - Purges melatonin in the system, as well as other unknown effects. Uses up all melatonin, the body's "nighttime" neurotransmitter. This is where it gets a bit iffy. If you smoke only during the night, your only withdrawal symptom should be insomnia and lack of dreams. If you smoke during the day, you are telling your body and brain that it is nighttime, when it isn't. This can lead to more withdrawal symptoms such as confusion, depression due to dismantled sleep cycles, and a general dislike of sunlight.

However, the damage is not permanent. Neurotransmitters are constantly being rebuilt. Some drug users, especially ecstasy users, follow a strict vitamin-enhanced diet. With plenty of all the essential vitamins and minerals in your system, your body can quickly rebuild the neurotransmitters, and refill the neurons. This is why some users feel withdrawal symptoms, while others do not. The vitamins essential for rebuilding melatonin are the B-complex vitamins. Neurotransmitters take a long time to rebuild (up to a week), so taking a vitamin pill won't help instantly.

Marijuana does damage in similar ways to addictive drugs, but is not permanent, unlike alcohol or disassociatives.