For Teenagers marijuana can lead to:

• School difficulties
• Problems with memory and concentration
• Increased aggression
• Car accidents
• Use of other drugs or alcohol
• Risky sexual behaviors
• Increased risk of suicide
• Increased risk of psychosis

Long-term use of marijuana can lead to:

• The same breathing problems as smoking cigarettes (coughing, wheezing, trouble with physical activity, and lung cancer)
• Decreased motivation or interest
• Lower intelligence
• Mental health problems, such as depression, anxiety, anger, moodiness, and psychosis
• Decreased or lack of response to mental health medication
• Increased risk of side effects from mental health medication

Their is no evidence that medical marijuana is any safer than any other.

http://www.aacap.org/cs/root/facts_for_families/marijuana_and_teens