From Harvard’s website:
In the short term, marijuana use impairs perception, judgment, thinking, memory, and learning; memory defects may persist six weeks after last use. Mental disorders connected with marijuana use merit their own category in the Diagnostic and Statistical Manual of Mental Disorders (DSM) IV, published by the American Psychiatric Association. These include Cannabis Intoxication (consisting of impaired motor coordination, anxiety, impaired judgment, sensation of slowed time, social withdrawal, and often includes perceptual disturbances; Cannabis Intoxication Delirium (memory deficit, disorientation); Cannabis Induced Psychotic Disorder, Delusions; Cannabis Induced Psychotic Disorder, Hallucinations; and Cannabis Induced Anxiety Disorder.
In addition, marijuana use has many indirect effects on health. Its effect on coordination, perception, and judgment means that it causes a number of accidents, vehicular and otherwise.