Ok. So here's the thing on marijuana.
Short term dangers: heart problems due to increased heart rate (50% increase), respiratory problems including suseptibility to infections, impaired judgements (MJ is a factor in one out of twenty fatal accidents), decreased testosterone and sperm count, loss of memory, flashes of paranoia.
Long term dangers: addiction, loss of interest (in work, school, relationships, etc.), lung cancer (3-5 times the danger of cigarettes), loss of ablity to remember (long term effects of THC storage), damage to unborn children (low weight, coordination difficulties) mental health issues (may trigger or increase).
The longer the use and the greater the use, the greater the effect. Some effects may disappear over time, others do not (i.e. cancer, triggered mental illness).