The use of marijuana can cause distorted perceptions, impaired coordination, difficulty in thinking and problem solving, and problems with learning and memory. Marijuana's effects on learning and memory can last for days or weeks after the high of the drug has worn off.
Long term effects include risk of lung disease and effects to your respiratory system which can cause asthma, colds, the flu and/or allergies to worsen. If you are developing and you are a guy, it can even affect your ability to have kids someday. Marijuana has been caused to decrease sperm count in men, as well as decrease sexual desire and increase the risk of health problems in adolescents.