Proponents of legalizing marijuana argue that it is a harmless drug.  Here are eight reasons listed by CNN.com that it is just not true.

1, However marijuana enters the body, the bloodstream carries THC to the brain, where it temporarily over activates parts of the mind involved in sensations, memory, movement, and other functions.

2. A steady increase in THC levels over the past few decades has made cannabis plants more potent than they used to be. This means new users are especially at risk of dangerous reactions to the drug. In recent years, annual emergency room visits related to marijuana use were estimated at more than 376,000.
3. A recent survey of middle and high school students found that an increasing number of kids believe pot is not dangerous. However, research shows that marijuana affects brain development. Heavy marijuana use in the teen years can cause problems with thinking, memory and learning that could become permanent.
4. Regular exposure to high doses of THC can lead to addiction. Research suggests that about 1 in 11 marijuana users become addicted, but these numbers are higher for those who use it daily and people who start in their teens.
5. Long-term use of marijuana has been linked to mental health problems, including depression and anxiety. In teens, it may be associated with suicidal thoughts. Some users experience temporary hallucinations and bouts of paranoia, and in people with schizophrenia, marijuana use can make symptoms worse.
6. If a pregnant woman uses marijuana, it may affect the developing brain of her fetus. Researchers have linked marijuana use during pregnancy to brain and behavioral problems in babies.
7. Marijuana doesn’t just go to your brain. Using marijuana sends THC circulating throughout the body. This increases your heart rate, raising the risk of a heart attack, especially in older adults and those with preexisting heart conditions.
8. Heavy marijuana smokers can have the same breathing problems as cigarette smokers. These include frequent coughing, excess phlegm, and a higher risk of lung infections. Researchers are not sure if marijuana smoking leads to lung cancer.