Common side effects:
Change of alertness
Impairment / alteration of short term memory
Dry eyes and mouth
Elevated heart rate
Coughing (which can lead to bronchitis)
Rare side effects:
Anxiety (most associated with smoking)
Paradoxical reaction (hyperactive, can’t sleep)
Amotivational syndrome (associated with frequent, large dosing over extended time)
Unstable heart disease
Driving or operating heavy machinery (not recommended)
The following areas need cautious consideration of the risks vs benefits.
Pregnancy / breast feeding; cannabis is associated with low birth weight when used in pregnancy and there is theoretical concern for hormonal disruption for male fetus
History of substance abuse
Unstable psychiatric problems
Respiratory problems / bronchitis (with smoking)
Under age 18
Long term effects:
Effects to the brain from chronic long term use is unclear in adults. Nothing clearly profound or debilitating has been proven so far.
The amount of time required to normalize from all effects of using cannabis is still not understood, but typically is 1-4 weeks depending on level of use. The CMCR has tried to figure this out with multiple methods. People with existing brain pathologies seem to be affected more than the rest of the population. The studies suggest that ill effects of cannabis diminish with time. Becoming dependent on cannabis before the age of 18 is associated with a drop in IQ by 6 points 20 years later. This drop in IQ concept however, is now soundly disputed. More info.
Here is balanced article on 12 negative effects of smoking cannabis