Just some food for thought when downplaying the seriousnss of marijuana
usage. These are just a few and do not include effects on the brain, etc.
Note the high percent in treatment centers. (322,000 is not an insignificant
number.) A strong indicator are the youth. If mom and dad can do it why can
I.
As a last point I don't see any admissions to treatment centers for usage of
coffee?

The latest treatment data indicate that in 2008 marijuana accounted for 17 percent of admissions (322,000) to treatment facilities in the United States, second only to opiates among illicit substances. Marijuana admissions were primarily male (74 percent), White (49 percent), and young (30 percent were in the 12-17 age range). Those in treatment for primary marijuana abuse had begun use at an early age: 56 percent by age 14.

Effects on the Heart
Marijuana increases heart rate by 20-100 percent shortly after smoking; this effect can last up to 3 hours. In one study, it was estimated that marijuana users have a 4.8-fold increase in the risk of heart attack in the first hour after smoking the drug.5 This may be due to increased heart rate as well as the effects of marijuana on heart rhythms, causing palpitations and arrhythmias. This risk may be greater in aging populations or in those with cardiac vulnerabilities.
Psychosis/Panic:

High doses of marijuana can cause psychosis or panic when you're high. Some people experience an acute psychotic reaction (disturbed perceptions and thoughts, paranoia) or panic attacks while under the influence of marijuana. This usually goes away as the drug's effects wear off. Scientists do not yet know if marijuana use causes lasting mental illness, although it can worsen psychotic symptoms in people who already have the mental illness schizophrenia, and it can increase the risk of long-lasting psychosis in those vulnerable to the disease.